- Fix custom ODB backends: acquire the GIL when calling back into Python,

and fix `write_cb`'s type argument

[#1496](https://github.com/libgit2/pygit2/pull/1496)

- Fix `References.get()`, `Branches.get()`, and the `in` operator on both

collections raising `InvalidSpecError` for invalid names; they now return

`None` and `False` respectively, as documented

[#1495](https://github.com/libgit2/pygit2/pull/1495)

- Fix `sh build.sh stubtest`

[#1492](https://github.com/libgit2/pygit2/pull/1492)

- Wheels: build the bundled libgit2 and libssh2 in Release mode instead of Debug

[#1497](https://github.com/libgit2/pygit2/pull/1497)