Fixes required for new release

[ianthomas23]

I am planning to get this into a state suitable for a new release. There are many test failures:

• Pandas extension array related failures.
• Parquet related failures (#84 may be relevant).
• Geometry intersection issues (#61 may be relevant).
• Shapely deprecation warnings, partly fixed by #85.

I am also planning to drop Python 3.6 support and add 3.9 and 3.10.

This is too much to do in a single PR so I will do multiple targetted PRs which will all be failing part of CI until they are all combined.

[jbednar]

Our hero!

[ianthomas23]

• Pandas extension array failures fixed by #88
• Parquet-related failures fixed by #92
• Geometry intersection tests failures fixed by #90
• Shapely deprecation warnings fixed by #85