Accept level names in any case

[sappelhoff]

setup_logging("warning") raised, because the stdlib only knows upper-case level names:

>>> pyprep.setup_logging("warning")
ValueError: Unknown level: 'warning'

mne.set_log_level("warning") accepts it, lower case is the spelling people reach for, and it
is what the sibling packages in this stack show in their docs. So the level is upper-cased when
it is a string, and passed through untouched when it is an int.

Tested for "warning", "WARNING", "Warning" and logging.WARNING.

pytest tests/test_logging.py: 57 passed. ruff check clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.92%. Comparing base (e252bf4) to head (bd3d17e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #210   +/-   ##
=======================================
  Coverage   97.92%   97.92%           
=======================================
  Files           8        8           
  Lines         869      869           
=======================================
  Hits          851      851           
  Misses         18       18           

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[sappelhoff]

Superseded by #211, which folds this level.upper() fix into a new _as_level helper as part of making pyprep quiet by default. The changelog entry is reworked there.