They were a feature for testing whose need was removed in
2015 (ua-parser/uap-core#58). They are *entirely* unnecessary, it is
possible a user somewhere is leveraging them for some reason.

So remove the override bits from the docstrings, and have them trigger
a `DeprecationWarning`.

Also:

- add tests that they do trigger
- remove support for them from the yaml testing functions
- update the pytest invocations to raise on all warnings
- while at it remove the apparently dead makePGTSComparisonYAML test
  utility method (?)
- and add pypy to the envlist

Note: `simplefilter` has to be updated to `"always"` for 2.7's dumb
      ass: the new tests triggers the same warning multiple times and
      of course `"default"` only yields them once.

Closes #119