Specify `"locale"` encoding when reading configuration file

Specify `"locale"` encoding when reading configuration file

Added ``encoding="locale"`` setting to the use of Python's
``ConfigParser.read()``, so that a warning is not generated when using the
recently added Python feature ``PYTHONWARNDEFAULTENCODING`` specified in
:pep:`597`. The encoding is passed as the ``"locale"`` string under Python
3.10 and greater, which indicates that the system-level locale should be
used, as was the case already here.  Pull request courtesy Kevin Kirsche.

Fixes: #1273
Closes: #1274
Pull-request: https://github.com/sqlalchemy/alembic/pull/1274
Pull-request-sha: d0abd33845754827b8ce69ba1360989c7c57fdb6

Change-Id: I75c1237ba8bd2e38890d0e5e4a578a6ca0b24883