Fix tests for membership to use 'not in'

flake8 reports:

  E713 test for membership should be 'not in'

Change-Id: I4446be67c431b7267105b53478d2ceba2af758d7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254451
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>

manifest_xml.py

@@ -1093,7 +1093,7 @@ class XmlManifest(object):
     diff = {'added': [], 'removed': [], 'changed': [], 'unreachable': []}
 
     for proj in fromKeys:
-      if not proj in toKeys:
+      if proj not in toKeys:
         diff['removed'].append(fromProjects[proj])
       else:
         fromProj = fromProjects[proj]