v0.42.0: Increment the version and add release notes.

Release 0.42.0

Major Features and Improvements

• Reduced memory usage for entropy compression.
• Updated com_google_protobuf version to v3.19.0.
• Removed dependency on six.

Breaking Changes

• Removed default value for the key parameter from the abstract base class
  tff.program.ReleaseManager.

Bug Fixes

• Fixed a whitespace syntax issue with shutting down a process when using the
  localhost C++ execution context.
• Modified tff.simulation.build_uniform_sampling_fn so that the output
  raises on non-integer inputs.
• Only wait a subprocess instance if it is not None.