Log Message:
py-pygit2: updated to 1.6.1

1.6.1
- Fix a number of reference leaks
- Review custom object backends

Breaking changes:
- In custom backends the callbacks have been renamed from ``read`` to
  ``read_cb``, ``write`` to ``write_cb``, and so on.

1.6.0
- New optional ``proxy`` argument in ``Remote`` methods
- New GIT_MERGE_PREFERENCE constants
- Don't require cached-property with Python 3.8 or later
- Add wheels for aarch64
- Documentation fixes
- Refactored build and CI, new ``build.sh`` script