Code improvements

- remove obsolete threading
- call getVersionHistory() as method not as function
  because we do not need the result here and use the internal
  variable "_revisions"
- simplify result for isEmpty()
- change "return None" to "return" which returns None per default
- PEP8 changes

Change-Id: I8eb71790f3c762feee1ff7d86fa91eed3be72b32
Merge "Code improvements"