packaging: don't crash building wix with python3.6 and earlier

`capture_output` was added in 3.7.  I was tempted to just check and abort in
build.py, since Windows doesn't have the Linux problem where some distros only
ship an older python.  But this is in a library that could be used elsewhere in
the future.

contrib/packaging/hgpackaging/util.py

@@ -1,157 +1,155 b''
 # util.py - Common packaging utility code.
 #
 # Copyright 2019 Gregory Szorc <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 # no-check-code because Python 3 native.
 
 import distutils.version
 import getpass
 import os
 import pathlib
 import subprocess
 import tarfile
 import zipfile
 
 
 def extract_tar_to_directory(source: pathlib.Path, dest: pathlib.Path):
     with tarfile.open(source, 'r') as tf:
         tf.extractall(dest)
 
 
 def extract_zip_to_directory(source: pathlib.Path, dest: pathlib.Path):
     with zipfile.ZipFile(source, 'r') as zf:
         zf.extractall(dest)
 
 
 def find_vc_runtime_files(x64=False):
     """Finds Visual C++ Runtime DLLs to include in distribution."""
     winsxs = pathlib.Path(os.environ['SYSTEMROOT']) / 'WinSxS'
 
     prefix = 'amd64' if x64 else 'x86'
 
     candidates = sorted(p for p in os.listdir(winsxs)
                   if p.lower().startswith('%s_microsoft.vc90.crt_' % prefix))
 
     for p in candidates:
         print('found candidate VC runtime: %s' % p)
 
     # Take the newest version.
     version = candidates[-1]
 
     d = winsxs / version
 
     return [
         d / 'msvcm90.dll',
         d / 'msvcp90.dll',
         d / 'msvcr90.dll',
         winsxs / 'Manifests' / ('%s.manifest' % version),
     ]
 
 
 def windows_10_sdk_info():
     """Resolves information about the Windows 10 SDK."""
 
     base = pathlib.Path(os.environ['ProgramFiles(x86)']) / 'Windows Kits' / '10'
 
     if not base.is_dir():
         raise Exception('unable to find Windows 10 SDK at %s' % base)
 
     # Find the latest version.
     bin_base = base / 'bin'
 
     versions = [v for v in os.listdir(bin_base) if v.startswith('10.')]
     version = sorted(versions, reverse=True)[0]
 
     bin_version = bin_base / version
 
     return {
         'root': base,
         'version': version,
         'bin_root': bin_version,
         'bin_x86': bin_version / 'x86',
         'bin_x64': bin_version / 'x64'
     }
 
 
 def find_signtool():
     """Find signtool.exe from the Windows SDK."""
     sdk = windows_10_sdk_info()
 
     for key in ('bin_x64', 'bin_x86'):
         p = sdk[key] / 'signtool.exe'
 
         if p.exists():
             return p
 
     raise Exception('could not find signtool.exe in Windows 10 SDK')
 
 
 def sign_with_signtool(file_path, description, subject_name=None,
                        cert_path=None, cert_password=None,
                        timestamp_url=None):
     """Digitally sign a file with signtool.exe.
 
     ``file_path`` is file to sign.
     ``description`` is text that goes in the signature.
 
     The signing certificate can be specified by ``cert_path`` or
     ``subject_name``. These correspond to the ``/f`` and ``/n`` arguments
     to signtool.exe, respectively.
 
     The certificate password can be specified via ``cert_password``. If
     not provided, you will be prompted for the password.
 
     ``timestamp_url`` is the URL of a RFC 3161 timestamp server (``/tr``
     argument to signtool.exe).
     """
     if cert_path and subject_name:
         raise ValueError('cannot specify both cert_path and subject_name')
 
     while cert_path and not cert_password:
         cert_password = getpass.getpass('password for %s: ' % cert_path)
 
     args = [
         str(find_signtool()), 'sign',
         '/v',
         '/fd', 'sha256',
         '/d', description,
     ]
 
     if cert_path:
         args.extend(['/f', str(cert_path), '/p', cert_password])
     elif subject_name:
         args.extend(['/n', subject_name])
 
     if timestamp_url:
         args.extend(['/tr', timestamp_url, '/td', 'sha256'])
 
     args.append(str(file_path))
 
     print('signing %s' % file_path)
     subprocess.run(args, check=True)
 
 
 PRINT_PYTHON_INFO = '''
 import platform; print("%s:%s" % (platform.architecture()[0], platform.python_version()))
 '''.strip()
 
 
 def python_exe_info(python_exe: pathlib.Path):
     """Obtain information about a Python executable."""
 
-    res = subprocess.run(
-        [str(python_exe), '-c', PRINT_PYTHON_INFO],
-        capture_output=True, check=True)
+    res = subprocess.check_output([str(python_exe), '-c', PRINT_PYTHON_INFO])
 
-    arch, version = res.stdout.decode('utf-8').split(':')
+    arch, version = res.decode('utf-8').split(':')
 
     version = distutils.version.LooseVersion(version)
 
     return {
         'arch': arch,
         'version': version,
         'py3': version >= distutils.version.LooseVersion('3'),
     }