Turn sqlalchemy.exc.Base20DeprecationWarning into errors in tests

To avoid introducing more warnings between now and a future PR that will
switch the future flag to turn these warnings into errors in prod, turn
these warning into errors in tests.

Author: amCap1712

pytest.ini

@@ -1,6 +1,6 @@
 [pytest]
 testpaths = brainzutils
-addopts = --cov-report html --cov=brainzutils -W always::DeprecationWarning
+addopts = --cov-report html --cov=brainzutils -W always::DeprecationWarning -W error::sqlalchemy.exc.Base20DeprecationWarning
 
 markers =
     database: requires access to the musicbrainz sample database