py-test: updated to 3.3.2

Pytest 3.3.2:

Bug Fixes
- pytester: ignore files used to obtain current user metadata in the fd leak
  detector.
- Fix **memory leak** where objects returned by fixtures were never destructed
  by the garbage collector.
- Fix conversion of pyargs to filename to not convert symlinks and not use
  deprecated features on Python 3.
- PYTEST_DONT_REWRITE is now checked for plugins too rather than only for
  test modules.

Improved Documentation
- Add clarifying note about behavior of multiple parametrized arguments

Trivial/Internal Changes
- Code cleanup.
- Clean up code by replacing imports and references of _ast to ast.

(adam)