setup: use old importlib_metadata version to fix kombu failing on python < 3.8

Many libraries use the importlib_metadata library as fallback when running on
Python versions older than 3.8 . For example setuptools when easy_install is
used for install the Kallithea console_scripts entrypoints in the bin folder.
The dependencies on importlib_metadata were indirect and without constrains on
version number.

The problem is that Celery uses Kombu, which (on Python < 3.8) uses
importlib_metadata in a way that is incompatible with importlib_metadata > 5.

Most obvious, building docs failed as:
  Running Sphinx v5.1.1

  Configuration error:
  There is a programmable error in your configuration file:

  Traceback (most recent call last):
    File ".../kallithea/venv/lib64/python3.7/site-packages/sphinx/config.py", line 347, in eval_config_file
      exec(code, namespace)
    File ".../kallithea/docs/conf.py", line 17, in <module>
      import kallithea
    File ".../kallithea/kallithea/__init__.py", line 45, in <module>
      CELERY_APP = celery.Celery()  # needed at import time but is lazy and can be configured later
    File ".../kallithea/venv/lib64/python3.7/site-packages/celery/local.py", line 492, in __getattr__
      [name])
    File ".../kallithea/venv/lib64/python3.7/site-packages/celery/app/__init__.py", line 2, in <module>
      from celery import _state
    File ".../kallithea/venv/lib64/python3.7/site-packages/celery/_state.py", line 15, in <module>
      from celery.utils.threads import LocalStack
    File ".../kallithea/venv/lib64/python3.7/site-packages/celery/utils/__init__.py", line 16, in <module>
      from .nodenames import nodename, nodesplit, worker_direct
    File ".../kallithea/venv/lib64/python3.7/site-packages/celery/utils/nodenames.py", line 6, in <module>
      from kombu.entity import Exchange, Queue
    File ".../kallithea/venv/lib64/python3.7/site-packages/kombu/entity.py", line 7, in <module>
      from .serialization import prepare_accept_content
    File ".../kallithea/venv/lib64/python3.7/site-packages/kombu/serialization.py", line 440, in <module>
      for ep, args in entrypoints('kombu.serializers'):  # pragma: no cover
    File ".../kallithea/venv/lib64/python3.7/site-packages/kombu/utils/compat.py", line 82, in entrypoints
      for ep in importlib_metadata.entry_points().get(namespace, [])
  AttributeError: 'EntryPoints' object has no attribute 'get'

That made readthedocs builds fail, when it in the default web configuration
used Python 3.7 .

Fixed by introducing an explicit dependency on importlib_metadata < 5.

setup.py

@@ -1,162 +1,164 b''
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 import os
 import platform
 import re
 import sys
 
 import setuptools
 # monkey patch setuptools to use distutils owner/group functionality
 from setuptools.command import sdist
 
 
 if sys.version_info < (3, 6):
     raise Exception('Kallithea requires Python 3.6 or later')
 
 
 here = os.path.abspath(os.path.dirname(__file__))
 
 
 def _get_meta_var(name, data, callback_handler=None):
     matches = re.compile(r'(?:%s)\s*=\s*(.*)' % name).search(data)
     if matches:
         s = eval(matches.groups()[0])
         if callable(callback_handler):
             return callback_handler(s)
         return s
 
 _meta = open(os.path.join(here, 'kallithea', '__init__.py'), 'r')
 _metadata = _meta.read()
 _meta.close()
 
 def callback(V):
     return '.'.join(map(str, V[:3])) + '.'.join(V[3:])
 __version__ = _get_meta_var('VERSION', _metadata, callback)
 __license__ = _get_meta_var('__license__', _metadata)
 __author__ = _get_meta_var('__author__', _metadata)
 __url__ = _get_meta_var('__url__', _metadata)
 # defines current platform
 __platform__ = platform.system()
 
 is_windows = __platform__ in ['Windows']
 
 requirements = [
     "alembic >= 1.0.10, < 1.5",
     "gearbox >= 0.1.0, < 1",
     "waitress >= 0.8.8, < 1.5",
     "WebOb >= 1.8, < 1.9",
     "backlash >= 0.1.2, < 1",
     "TurboGears2 >= 2.4, < 2.5",
     "tgext.routes >= 0.2.0, < 1",
     "Beaker >= 1.10.1, < 2",
     "WebHelpers2 >= 2.0, < 2.1",
     "FormEncode >= 1.3.1, < 2.1",
     "SQLAlchemy >= 1.2.9, < 1.4",
     "Mako >= 0.9.1, < 1.2",
     "Pygments >= 2.2.0, < 2.7",
     "Whoosh >= 2.7.1, < 2.8",
     "celery >= 5, < 5.1",
     "Babel >= 1.3, < 2.9",
     "python-dateutil >= 2.1.0, < 2.9",
     "Markdown >= 2.2.1, < 3.2",
     "docutils >= 0.11, < 0.17",
     "URLObject >= 2.3.4, < 2.5",
     "Routes >= 2.0, < 2.5",
     "dulwich >= 0.19.0, < 0.20",
     "mercurial >= 5.2, < 6.2",
     "decorator >= 4.2.1, < 4.5",
     "Paste >= 2.0.3, < 3.5",
     "bleach >= 3.2, < 4.2",
     "Click >= 7.0, < 8",
     "ipaddr >= 2.2.0, < 2.3",
     "paginate >= 0.5, < 0.6",
     "paginate_sqlalchemy >= 0.3.0, < 0.4",
     "bcrypt >= 3.1.0, < 3.2",
     "pip >= 20.0, < 999",
     "chardet >= 3",
 ]
+if sys.version_info < (3, 8):
+    requirements.append("importlib-metadata < 5")
 
 dependency_links = [
 ]
 
 classifiers = [
     'Development Status :: 4 - Beta',
     'Environment :: Web Environment',
     'Framework :: Pylons',
     'Intended Audience :: Developers',
     'License :: OSI Approved :: GNU General Public License (GPL)',
     'Operating System :: OS Independent',
     'Programming Language :: Python :: 3.6',
     'Programming Language :: Python :: 3.7',
     'Programming Language :: Python :: 3.8',
     'Topic :: Software Development :: Version Control',
 ]
 
 
 # additional files from project that goes somewhere in the filesystem
 # relative to sys.prefix
 data_files = []
 
 description = ('Kallithea is a fast and powerful management tool '
                'for Mercurial and Git with a built in push/pull server, '
                'full text search and code-review.')
 
 keywords = ' '.join([
     'kallithea', 'mercurial', 'git', 'code review',
     'repo groups', 'ldap', 'repository management', 'hgweb replacement',
     'hgwebdir', 'gitweb replacement', 'serving hgweb',
 ])
 
 # long description
 README_FILE = 'README.rst'
 try:
     long_description = open(README_FILE).read()
 except IOError as err:
     sys.stderr.write(
         "[WARNING] Cannot find file specified as long_description (%s): %s\n"
         % (README_FILE, err)
     )
     long_description = description
 
 
 sdist_org = sdist.sdist
 class sdist_new(sdist_org):
     def initialize_options(self):
         sdist_org.initialize_options(self)
         self.owner = self.group = 'root'
 sdist.sdist = sdist_new
 
 packages = setuptools.find_packages(exclude=['ez_setup'])
 
 setuptools.setup(
     name='Kallithea',
     version=__version__,
     description=description,
     long_description=long_description,
     keywords=keywords,
     license=__license__,
     author=__author__,
     author_email='[e-mail]',
     dependency_links=dependency_links,
     url=__url__,
     install_requires=requirements,
     classifiers=classifiers,
     data_files=data_files,
     packages=packages,
     include_package_data=True,
     message_extractors={'kallithea': [
             ('**.py', 'python', None),
             ('templates/**.mako', 'mako', {'input_encoding': 'utf-8'}),
             ('templates/**.html', 'mako', {'input_encoding': 'utf-8'}),
             ('public/**', 'ignore', None)]},
     zip_safe=False,
     entry_points="""
     [console_scripts]
     kallithea-api =    kallithea.bin.kallithea_api:main
     kallithea-gist =   kallithea.bin.kallithea_gist:main
     kallithea-cli =    kallithea.bin.kallithea_cli:cli
 
     [paste.app_factory]
     main = kallithea.config.application:make_app
     """,
 )