Overhaul type hints with beartype and PEP 585 compliance

[MilesCranmer]

This implements the following:

1. Sets up beartype runtime type checking for the entire package
   • I use the configuration that type errors => warnings. I don't want to break anything if a type is incorrect but still works; just want to let the user know a type is unexpected.
   • This runtime type checking seems pretty fast, so speed shouldn't be an issue.
   • (thanks @PatrickKidger for mentioning beartype; it is lovely!)
   • BearType is not available on Python <3.8 so I only include the install on Python 3.8+
2. Updates type symbols to the PEP 585 standards.
3. Moves various stages of processing the data (like converting pandas -> numpy; getting column names; feature selection) into dataclasses so improve readability/robustness. The dataclasses have frozen=True so I can be sure they are not modified by other functions.

TODO:

• Only use beartype during testing rather than during runtime.
• Possibly revert | as shorthand for Union or see if there's any way to get that behavior from future
• Implement a from future import __annotations__ call where the new type syntax is used

[MilesCranmer]

Still not sure this should be on by default or not... Perhaps we just set it up for testing.