Work around a mypy bug with Python 3.11

Mypy (many versions) raises a wrong error for Python 3.11:

  pgactivity/types.py:888: error: Returning Any from function declared to return "str"  [no-any-return]

Should be fixed by commit:

  python/mypy@05a3f7d

pgactivity/types.py

@@ -885,6 +885,8 @@ class LockType(enum.Enum):
 
     def __str__(self) -> str:
         # Custom str(self) for transparent rendering in views.
+        assert isinstance(self.name, str)
+        # TODO ^ remove this assert for mypy > 0.991
         return self.name