Update databases/py-cassandra-driver to 3.8.0.

Features
- Quote index names in metadata CQL generation (PYTHON-616)
- On column deserialization failure, keep error message consistent
  between python and cython (PYTHON-631)
- TokenAwarePolicy always sends requests to the same replica for a given
  key (PYTHON-643)
- Added cql types to result set (PYTHON-648)
- Add __len__ to BatchStatement (PYTHON-650)
- Duration Type for Cassandra (PYTHON-655)
- Send flags with PREPARE message in v5 (PYTHON-684)

Bug Fixes
- Potential Timing issue if application exits prior to session pool
  initialization (PYTHON-636)
- "Host X.X.X.X has been marked down" without any exceptions
  (PYTHON-640)
- NoHostAvailable or OperationTimedOut when using execute_concurrent
  with a generator that inserts into more than one table (PYTHON-642)
- ResponseFuture creates Timers and don't cancel them even when result
  is received which leads to memory leaks (PYTHON-644)
- Driver cannot connect to Cassandra version > 3 (PYTHON-646)
- Unable to import model using UserType without setuping connection
  since 3.7 (PYTHON-649)
- Don't prepare queries on ignored hosts on_up (PYTHON-669)
- Sockets associated with sessions not getting cleaned up on
  session.shutdown() (PYTHON-673)
- Make client timestamps strictly monotonic (PYTHON-676)
- cassandra.cqlengine.connection.register_connection broken when
  hosts=None (PYTHON-692)

Other
- Create a cqlengine doc section explaining None semantics (PYTHON-623)
- Resolve warnings in documentation generation (PYTHON-645)
- Cython dependency (PYTHON-686)
- Drop Support for Python 2.6 (PYTHON-690)

(fhajny)