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