setup: fix the py2exe logic to work with py3

TortoiseHg still uses (the modernized) py2exe packaging, but the build was
failing since `py2exe.Distribution` was removed.

One thing to note is that later in this module, there's a hack to include
`distutils` when building from a virtualenv.  While `import distutils` works in
`hg debugshell` when built with py2, it doesn't work in py3.  I'm not sure why-
I don't see it in `library.zip` either.  It doesn't seem to break anything
though.

Differential Revision: https://phab.mercurial-scm.org/D12553

setup.py

@@ -197,7 +197,7 b' def hasheader(cc, headername):'
 try:
     import py2exe
 
-    py2exe.Distribution  # silence unused import warning
+    py2exe.patch_distutils()
     py2exeloaded = True
     # import py2exe's patched Distribution class
     from distutils.core import Distribution