Log Message:
databases/py-cassandra-driver: Update to 3.15.0

Features
--------
- Parse Virtual Keyspace Metadata

Bug Fixes
---------
- Tokenmap.get_replicas returns the wrong value if token coincides
  with the end of the range
- Python Driver fails with "more than 255 arguments" python exception
  when > 255 columns specified in query response
- Hang in
  integration.standard.test_cluster.ClusterTests.test_set_keyspace_twice
- Asyncore reactors should use a global variable instead of a class
  variable for the event loop

Other
-----
- Use global variable for libev loops so it can be subclassed
- Update SchemaParser for V4
- Bump Cython dependency version to 0.28