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

@@ -81,8 +81,9 @@ class CASCache(ArtifactCache):
     ################################################
 
     def preflight(self):
-        if (not os.path.isdir(os.path.join(self.casdir, 'refs', 'heads')) or
-            not os.path.isdir(os.path.join(self.casdir, 'objects'))):
+        headdir = os.path.join(self.casdir, 'refs', 'heads')
+        objdir = os.path.join(self.casdir, 'objects')
+        if not (os.path.isdir(headdir) and os.path.isdir(objdir)):
             raise ArtifactError("CAS repository check failed for '{}'"
                                 .format(self.casdir))
 
@@ -918,7 +919,7 @@ class CASCache(ArtifactCache):
             # Skip download, already in local cache.
             pass
         elif (digest.size_bytes >= remote.max_batch_total_size_bytes or
-                not remote.batch_read_supported):
+              not remote.batch_read_supported):
             # Too large for batch request, download in independent request.
             self._ensure_blob(remote, digest)
             in_local_cache = True