Pygit2 0.21.2 fixes regression, use it with libgit2 0.21.1

This release fixes a regression introduced in version 0.21.0, with the index and Python 2. See the full list of changes.

Thanks to Carlos Martín Nieto and Arno van Lumig.

Otherwise most important is the maintenance release of libgit2 v0.21.1, which among other corrections fixes a critical bug which may lead to data corruption. See the full list of libgit2 v021.1 changes.

So upgrading to libgit2 0.21.1 and pygit2 0.21.2 is highly recommended.