Drop Python 3.7 support

[lemon24]

Bokeh seems to be preparing to do so, maybe we should start preparing too.

Off the top of my head, the oldest Python version reader should support is (maybe it's a good idea to add this to the docs somewhere):

• the newest available on the latest Ubuntu LTS (3 months after the LTS is release)
• ... including at least 1 stable PyPy version

As of Match 2020, Ubuntu 22.04 seems to have CPython 3.10 and PyPy 3.8.

Things reader can take advantage of after dropping 3.7:

• assignment expressions :=
• positional-only parameters (might be a good idea to do this in #268)
• don't need to vendor functools.cached_property anymore
• typing: Literal, TypedDict, Protocol, Final