tox: Don't delete Python3 .pyc files
tox: Don't delete Python3 .pyc files

Python3 is smarter than Python2 about when to make use of .pyc files, so
there's no need to remove them before running unit tests just in case the
developer has switched branches.[1] To save recompiling before every test
run, disable this behaviour for Python3.

Also, use a consistent find command in all of the environments where we do
this.

[1] https://www.python.org/dev/peps/pep-3147/#case-3-pycache-foo-magic-pyc-with-no-source

Change-Id: Iabaa4a697329e7538fab93541f41fc208e591382

openstack/heat,noironetworks/heat,openstack/heat,noironetworks/heat