Fix test and LGTM warnings.

- Fixes LGTM warnings for an unused import and equality comparisons to
  None in SQLAlchemy filters.
- Removes part of a unit test asserting that the claimed_until locking
  mechanism works correctly. If I recall correctly, this does not work
  in unit tests because the test takes place inside a transaction, and
  the database provider does evaluate the current time until the
  transaction is written.

alembic/versions/07e0598199f6_add_applications_claimed_until.py

@@ -7,7 +7,6 @@ Create Date: 2020-01-25 13:33:17.711548
 """
 from alembic import op
 import sqlalchemy as sa
-from sqlalchemy.dialects import postgresql
 
 # revision identifiers, used by Alembic.
 revision = '07e0598199f6' # pragma: allowlist secret