py-test: updated to 3.7.3

pytest 3.7.3:

Bug Fixes
* Fixtures during teardown can again use capsys and capfd to inspect output captured during tests.
* Fix collection of tests from __init__.py files if they match the python_files configuration option.
* Fix issue where teardown of fixtures of consecutive sub-packages were executed once, at the end of the outer package.
* Fix bug where --show-capture=no option would still show logs printed during fixture teardown.
* Fix stdout/stderr not getting captured when real-time cli logging is active.
* Fix collection error when specifying test functions directly in the command line using test.py::test syntax together with --doctest-modules.
* Fix bugs where unicode arguments could not be passed to testdir.runpytest on Python 2.
* Fix double collection of tests within packages when the filename starts with a capital letter.

(adam)