py-test: updated to 3.6.1

Pytest 3.6.1:
Bug Fixes
* Fixed a bug where stdout and stderr were logged twice by junitxml when a test was marked xfail.
* Fix usefixtures mark applyed to unittest tests by correctly instantiating FixtureInfo.
* Fix assertion rewriter compatibility with libraries that monkey patch file objects.

Improved Documentation
* Added a section on how to use fixtures as factories to the fixture documentation.

Trivial/Internal Changes
* Enable caching for pip/pre-commit in order to reduce build time on travis/appveyor.
* Switch pytest to the src/ layout as we already suggested it for good practice - now we implement it as well.
* Fix if in tests to support 3.7.0b5, where a docstring handling in AST got reverted.
* Remove some python2.5 compatibility code.

(adam)