add slotscheck to CI

add slotscheck to CI

As discussed in #7589, `slotscheck` can prevent slots-related mistakes from creeping back in.

Plan for now is to have slotscheck part of the "lint" tests
(renamed from pep8) that will run for CI and github actions.

To support slotscheck's runtime nature, slotscheck is
run twice, first with cython exts enabled and then
with them disabled via new environment variable.
Also added sqlalchemy[mypy] dependency to support slots
checking the mypy plugin.

Found and fixed one more `__slots__` issue by disabling C
exts.

Closes: #7670
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7670
Pull-request-sha: 3e77fe5449615a3c7c61ce9a1e4e79cd6636a89a

Change-Id: I90cdd284cdcee316a38856ba94d72ffc98947c5a