Fix lint issues

Description

There are a large number of pylint related issues raised by newer pylint versions. This MR attempts to address them.

In order to make it easier to deal with adding/removing changes, there are a large number of commits in this MR, though they may be partially squashed together toward the end of things.

In addition, there's one or two minor bugs which show up as part of the linting effort. These are also corrected here.

buildstream/_artifactcache/cascache.py

@@ -37,7 +37,6 @@ from .._protos.google.bytestream import bytestream_pb2, bytestream_pb2_grpc
 from .._protos.build.bazel.remote.execution.v2 import remote_execution_pb2, remote_execution_pb2_grpc
 from .._protos.buildstream.v2 import buildstream_pb2, buildstream_pb2_grpc
 
-from .._message import MessageType, Message
 from .. import _signals, utils
 from .._exceptions import ArtifactError