Drop Python 3.9 support

[adamjstewart]

It's that time of year again!

We've been following NEP 29 for a while now to decide when to drop support for older Python versions. However, NEP 29 is now superseded by SPEC 0. Although NEP 29 adopted a 3.5 year support strategy, SPEC 0 now adopts a shorter 3 year support strategy. Since a new Python minor release happens once a year, this means support for the 3 most recent Python minor versions. However, PyTorch is slow to adopt the latest Python version after it gets released, so I propose waiting until it is fully supported before dropping the oldest version.

The biggest change in Python 3.10 is support for X | Y as a replacement for typing.Union[X, Y] and X | None as a replacement for Optional[X]. This means that typing no longer needs to be imported by most files! It's also an excuse to bump the minimum supported version for many of our dependencies.

[adamjstewart]

Can also use the new match-case statement introduced in 3.10 in places like DataModule.setup.

This is an if expression, not an elif expression. And Python match statements do not support fall-through.

[isaaccorley]

I double checked each change and looks good