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/utils.py

@@ -29,13 +29,13 @@ import re
 import shutil
 import signal
 import stat
+from stat import S_ISDIR
 import string
 import subprocess
 import tempfile
 import itertools
 import functools
 from contextlib import contextmanager
-from stat import S_ISDIR
 
 import psutil