drop 3.7, add mypy test, bump reqs, etc

[jakkdl]

Split out from #188 with a --patch checkout. This does destroy the commit history, so I might need to rewrite the history in #188 to avoid conflicts.

• Drops python3.7 support. It still works afaik, but is a pain to test in CI.
• Adds loose mypy run to Makefile & CI
• Adds some type annotations
• Adds 3.13-dev test run. It currently fails, but will pass once trio and cffi push new releases.
• Adds blurb to readme on status of the project.
• Bump package versions in requirements-dev.txt and requirements-dev-full.txt
• Adds small tests to slightly bump code coverage