RDFlib 6.1.1

Better testing and tidier code.

This is a semi-major release that:

• add support for Python 3.10
• updates the test suite to pytest (from nose)
• tidies up a lot of continuous integration
• gets more tests tested, not skipped
• implements lots of mypy tests
• updates several parsers and serializers
  • supports the new HexTuples format!
• many bug fixes

This release contains many, many hours of updates from Iwan Aucamp, so thank you Iwan!