Delete python bytecode before every test run

Because python creates pyc files during tox runs, certain
changes in the tree, like deletes of files, or switching
branches, can create spurious errors.

Change-Id: Id8798992dd71d9a7b16cba9e822bab236f0fcad7
Closes-Bug: #1368661

tox.ini

@@ -9,11 +9,13 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = pip install pymongo===3.0.3
+commands = find . -type f -name "*.pyc" -delete
+           pip install pymongo===3.0.3
            {envpython} run_tests.py
            python setup.py testr --slowest
            {envpython} generate_examples.py
 whitelist_externals = bash
+                      find
 
 [tox:jenkins]
 sitepackages = True