Pybase64

- Release the GIL (293, 300)
- Publish CPython 3.10 wheels
- Drop python 3.5 support

- Add macOS arm64 wheel

- GitHub Actions: fix build on tag

- Add PyPy wheels
- Add aarch64, ppc64le & s390x manylinux wheels

- Move CI from TravisCI/AppVeyor to GitHub Actions
- Fix publication of Linux/macOS wheels

- Add b64encode_as_string, same as b64encode but returns a str object instead of a bytes object
- Add b64decode_as_bytearray, same as b64decode but returns a bytarray object instead of a bytes object
- Speed-Up decoding from UCS1 strings