setup: exclude the git extension from py2 builds

This can't be built on Windows with the py2 compiler, and while old versions can
be installed via pip on Linux, I can't get the tests to run (even with py3.8)
using pygit2 0.28.2.  Some manually run commands work, and others spew stack
traces that don't occur with the current 1.4.0 release using py3.

Differential Revision: https://phab.mercurial-scm.org/D9604

setup.py

@@ -1309,6 +1309,13 b' packages = ['
     'hgdemandimport',
 ]
 
+# The pygit2 dependency dropped py2 support with the 1.0 release in Dec 2019.
+# Prior releases do not build at all on Windows, because Visual Studio 2008
+# doesn't understand C 11.  Older Linux releases are buggy.
+if sys.version_info[0] == 2:
+    packages.remove('hgext.git')
+
+
 for name in os.listdir(os.path.join('mercurial', 'templates')):
     if name != '__pycache__' and os.path.isdir(
         os.path.join('mercurial', 'templates', name)