Bug 1253359 - Vendor in Pytest 2.9.2 and Py 1.4.31 r=gps

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

python/pytest/_pytest/_pluggy.py

@@ -0,0 +1,11 @@
+"""
+imports symbols from vendored "pluggy" if available, otherwise
+falls back to importing "pluggy" from the default namespace.
+"""
+
+try:
+    from _pytest.vendored_packages.pluggy import *  # noqa
+    from _pytest.vendored_packages.pluggy import __version__  # noqa
+except ImportError:
+    from pluggy import *  # noqa
+    from pluggy import __version__  # noqa