Update databases/py-cassandra-driver to 3.9.0.

3.9.0

Features
- cqlengine: remove elements by key from a map

Bug Fixes
- improve error handling when connecting to non-existent keyspace
- Sockets associated with sessions not getting cleaned up on
  session.shutdown()
- rare flake on
  integration.standard.test_cluster.ClusterTests.test_clone_shared_lbp
- MontonicTimestampGenerator.__init__ ignores class defaults
- race where callback or errback for request may not be called
- cqlengine: model.update() should not update columns with a default
  value that hasn't changed
- cqlengine: field value manager's explicit flag is True when queried
  back from cassandra

Other
- Connection not closed in example_mapper
- Remove mention of pre-2.0 C* versions from OSS 3.0+ docs

3.8.1

Bug Fixes
- implement __le__/__ge__/__ne__ on some custom types
- Fix bug in eventlet and gevent reactors that could cause hangs
- Fix DecimalType regression

(fhajny)