Mypy stubs and general maintenance

[cdgriffith]

• Adding testing for Python 3.10-dev
• Changing to use tomli instead of toml by default as it is faster and toml 1.0.0 compatible
• Fixing Fix mappings for mypy >= 0.8 (shows tests failing) #189 by adding mappings for mypy

[DanieleQ97]

About tomli, Python 3.11 should add tomllib which is basically tomli but built-in.

Maybe in future toml could be dropped entirely and the import changed to something like:

try:
    import tomllib
except ModuleNotFoundError:
    import tomli as tomllib

[cdgriffith]

https://docs.python.org/3.11/library/tomllib.html

This module does not support writing TOML.

Will have to wait until it works for both reading and writing before it would be worth implementing IMO. And then will have to do a check for versions like this 3.11 for when it doesn't support writing. Them releasing it this early without writing abilities is quite a shortsighted headace.