Remove support for Python 3.6

See #24.

This also enables support for Python 3.11.

Rust rewrite

Moved away from cffi to use grammers-crypto as the backing implementation and pyo3 in order to provide Python bindings.

cffi rewrite

Stopped using C / C++ Python Extensions for CPython in favour of cffi, which is the correct way of building portable C extensions for different interpreters.

Now requires libffi-dev for building. For CPython, python3-dev is also necessary.