Pygit2

- Upgrade to libgit2 v1.4
- Documentation, new recipes for committing and cloning
[1125](https://github.com/libgit2/pygit2/pull/1125)

- Rename `RemoteCallbacks.progress(...)` callback to
`.sideband_progress(...)`
[1120](https://github.com/libgit2/pygit2/pull/1120)
- New `Repository.merge_base_many(...)` and
`Repository.merge_base_octopus(...)`
[1112](https://github.com/libgit2/pygit2/pull/1112)
- New `Repository.listall_stashes()`
[1117](https://github.com/libgit2/pygit2/pull/1117)
- Code cleanup [1118](https://github.com/libgit2/pygit2/pull/1118)

Backward incompatible changes:

- The `RemoteCallbacks.progress(...)` callback has been renamed to
`RemoteCallbacks.sideband_progress(...)`. This matches the
documentation, but may break existing code that still uses the old
name.

- Universal wheels for macOS
[1109](https://github.com/libgit2/pygit2/pull/1109)

- New `Repository.amend_commit(...)`
[1098](https://github.com/libgit2/pygit2/pull/1098)
- New `Commit.message_trailers`
[1101](https://github.com/libgit2/pygit2/pull/1101)
- Windows wheels for Python 3.10
[1103](https://github.com/libgit2/pygit2/pull/1103)
- Changed: now `DiffDelta.is_binary` returns `None` if the file data
has not yet been loaded, cf.
[962](https://github.com/libgit2/pygit2/issues/962)
- Document `Repository.get_attr(...)` and update theme
[1017](https://github.com/libgit2/pygit2/issues/1017)
[1105](https://github.com/libgit2/pygit2/pull/1105)

- Upgrade to libgit2 1.3.0
[1089](https://github.com/libgit2/pygit2/pull/1089)
- Linux wheels now bundled with libssh2 1.10.0 (instead of 1.9.0)
- macOS wheels now include libssh2
- Add support for Python 3.10
[1092](https://github.com/libgit2/pygit2/pull/1092)
[1093](https://github.com/libgit2/pygit2/pull/1093)
- Drop support for Python 3.6
- New [pygit2.GIT_CHECKOUT_SKIP_LOCKED_DIRECTORIES]{.title-ref}
[1087](https://github.com/libgit2/pygit2/pull/1087)
- New optional argument `location` in `Repository.applies(..)` and
`Repository.apply(..)`
[1091](https://github.com/libgit2/pygit2/pull/1091)
- Fix: Now the [flags]{.title-ref} argument in
[Repository.blame()]{.title-ref} is passed through
[1083](https://github.com/libgit2/pygit2/pull/1083)
- CI: Stop using Travis, move to GitHub actions

Caveats:

- Windows wheels for Python 3.10 not yet available.

- 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.