…t 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

…m within exceptiongroups

revert making close_connection CS shielded, as that would be a behaviour change causing very long stalls with the default timeout of 60s

add comment for pylint disable

move RaisesGroup import

…oups_support

…le cases