Log Message:
Update databases/py-cassandra-driver to 3.1.0.

Features
- Pass name of server auth class to AuthProvider
- Surface schema agreed flag for DDL statements
- Automatically convert float and int to Decimal on serialization
- Eventlet Reactor IO improvement
- Make pure Python ProtocolHandler available even when Cython is
  present
- Optional Cython deserializer for bytes as bytearray
- Add Session.default_serial_consistency_level
- cqlengine: Expose prior state information via cqlengine
  LWTException
- cqlengine: Collection datatype "contains" operators support
  (Cassandra 2.1)
- cqlengine: Add DISTINCT query operator
- cqlengine: Tuple cqlengine api
- cqlengine: Add support for UPDATE/DELETE ... IF EXISTS statements
- cqlengine: Allow nested container types
- cqlengine: Add ability to set query's fetch_size and limit
- cqlengine: Internalize default keyspace from successive set_session
- cqlengine: Warn when Model.create() on Counters

Bug Fixes
- Bus error (alignment issues) when running cython on some ARM
  platforms
- Overflow when decoding large collections (cython)
- Timer heap comparison issue with Python 3
- Cython deserializer date overflow at 2^31 - 1
- Decode error encountered when cython deserializing large map
  results
- Don't require Cython for build if compiler or Python header
  not present
- Unorderable types in task scheduling with Python 3
- cqlengine: Fix crash when updating a UDT column with a None value
- cqlengine: Race condition in ..connection.execute with lazy_connect
- cqlengine: doesn't support case sensitive column family names
- cqlengine: UserDefinedType mandatory in create or update
- cqlengine: db_field breaks UserType
- cqlengine: UDT badly quoted
- cqlengine: Use of db_field on primary key prevents querying except
  while tracing.
- cqlengine: DateType.deserialize being called with one argument
  vs two
- cqlengine: Querying without setting up connection now throws
  AttributeError and not CQLEngineException
- cqlengine: BatchQuery multiple time executing execute statements.
- cqlengine: Better error for management functions when no connection
  set
- cqlengine: Handle None values for UDT attributes in cqlengine
- cqlengine: Fix inserting None for model save
- cqlengine: EQ doesn't map to a QueryOperator (setup race condition)
- cqlengine: class.MultipleObjectsReturned has DoesNotExist as base
  class
- cqlengine: Typo in cqlengine UserType __len__ breaks attribute
  assignment

Other
- cqlengine: a major improvement on queryset has been introduced.
  It is a lot more efficient to iterate large datasets: the rows
  are now fetched on demand using the driver pagination.
- cqlengine: the queryset len() and count() behaviors have changed.
  It now executes a "SELECT COUNT(*)" of the query rather than
  returning the size of the internal result_cache (loaded rows).
  On large queryset, you might want to avoid using them due to
  the performance cost. Note that trying to access objects using
  list index/slicing with negative indices also requires a count
  to be executed.