Python enhancement follow ups

[oschaaf]

Creating this issue as a sink for leftovers from #395

• We still don't show unused imports because of pytest peculiarities. There are unused imports however,
  those should be removed, and it would be good to look into suppressing false positives at the source code level
  so we can enable the check.
• The doc strings for return values may not be formatted according to the desired spec (they mention the type).
  Revisit that, and see if we need to exclude a check and/or amend these doc strings.
• test/integration/utility.py has a too unspecific name. We should move its contents into more specifically named
  modules.
• Currently we have our own assertation helpers. If we can just use https://docs.python.org/2/library/unittest.html we should do that, so we can eliminate our own.