Vendor in Pytest (2.9.2) and its requirement Py (1.4.31),
so that it can be used for e.g. the Marionette
harness unit tests and a pytest plugin for mozlog.
Copy pytest and py package directories (extracted from
tars on Pip) into `mozilla-central/python/`, removing
some support files (e.g. changelog, docs, tests).
Add both `.pth` entries to `virtualenv_packages.txt`.
Add both paths to `SEARCH_PATHS` in `mach_bootstrap.py`.
MozReview-Commit-ID: IOTCOUxX8R9
