demandimport: blacklist distutils.msvc9compiler (issue4475)

This module depends on _winreg, which is windows-only. Recent versions
of setuptools load distutils.msvc9compiler and expect it to
ImportError immediately when on non-Windows platforms, so we need to
let them do that. This breaks in an especially mystifying way, because
setuptools uses vars() on the imported module. We then throw an
exception, which vars doesn't pick up on well. For example:

In [3]: class wat(object):
   ...:     @property
   ...:     def __dict__(self):
   ...:         assert False
   ...:

In [4]: vars(wat())
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-2781ada5ffe6> in <module>()
----> 1 vars(wat())

TypeError: vars() argument must have __dict__ attribute

Which is similar to the problem we run into.

mercurial/demandimport.py

@@ -164,6 +164,8 b' ignore = ['
     '_ssl', # conditional imports in the stdlib, issue1964
     'rfc822',
     'mimetools',
+    # setuptools 8 expects this module to explode early when not on windows
+    'distutils.msvc9compiler'
     ]
 
 def isenabled():

tests/test-demandimport.py

@@ -1,6 +1,16 b''
 from mercurial import demandimport
 demandimport.enable()
 
+import os
+if os.name != 'nt':
+    try:
+        import distutils.msvc9compiler
+        print ('distutils.msvc9compiler needs to be an immediate '
+               'importerror on non-windows platforms')
+        distutils.msvc9compiler
+    except ImportError:
+        pass
+
 import re
 
 rsub = re.sub