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/sandbox/_mount.py

@@ -30,7 +30,7 @@ from .._fuse import SafeHardlinks
 # Helper data object representing a single mount point in the mount map
 #
 class Mount():
-    def __init__(self, sandbox, mount_point, safe_hardlinks, fuse_mount_options={}):
+    def __init__(self, sandbox, mount_point, safe_hardlinks, fuse_mount_options=None):
         scratch_directory = sandbox._get_scratch_directory()
         # Getting _get_underlying_directory() here is acceptable as
         # we're part of the sandbox code. This will fail if our
@@ -39,7 +39,7 @@ class Mount():
 
         self.mount_point = mount_point
         self.safe_hardlinks = safe_hardlinks
-        self._fuse_mount_options = fuse_mount_options
+        self._fuse_mount_options = {} if fuse_mount_options is None else fuse_mount_options
 
         # FIXME: When the criteria for mounting something and its parent
         #        mount is identical, then there is no need to mount an additional
@@ -101,10 +101,13 @@ class Mount():
 #
 class MountMap():
 
-    def __init__(self, sandbox, root_readonly, fuse_mount_options={}):
+    def __init__(self, sandbox, root_readonly, fuse_mount_options=None):
         # We will be doing the mounts in the order in which they were declared.
         self.mounts = OrderedDict()
 
+        if fuse_mount_options is None:
+            fuse_mount_options = {}
+
         # We want safe hardlinks on rootfs whenever root is not readonly
         self.mounts['/'] = Mount(sandbox, '/', not root_readonly, fuse_mount_options)