bpo-44630: Fix assertion errors in csv module (GH-27127)

bpo-44630: Fix assertion errors in csv module (GH-27127)

Fix incorrect handling of exceptions when interpreting dialect objects in
the csv module. Not clearing exceptions between calls to
PyObject_GetAttrString() causes assertion failures in pydebug mode (or with
assertions enabled).

Add a minimal test that would've caught this (passing None as dialect, or
any object that isn't a csv.Dialect subclass, which the csv module allows
and caters to, even though it is not documented.) In pydebug mode, the test
triggers the assertion failure in the old code.

Contributed-By: T. Wouters [Google]
(cherry picked from commit 0093876328afa330224c9d887c18dee0b3117852)

Co-authored-by: T. Wouters <thomas@python.org>