cleanup pyc files between running pytest

this is need to avoid problem sometimes tests are run inside docker
container and sometimes local.

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/_pytest/config/__init__.py", line 409, in _importconftest
mod = conftestpath.pyimport()
File "/usr/lib/python2.7/dist-packages/py/_path/local.py", line 686, in pyimport
raise self.ImportMismatchError(modname, modfile, self)
ImportMismatchError: ('conftest', '/beremiz/tests/tools/conftest.py', local('/home/developer/beremiz/tests/tools/conftest.py'))
ERROR: could not load /home/developer/beremiz/tests/tools/conftest.py

# those files are used in runtime
# licensed according to LGPL, see COPYING.runtime 

images/icostop24.png
images/icoplay24.png
images/brz.png
util/__init__.py
util/paths.py
runtime/WampClient.py
runtime/PLCObject.py
runtime/NevowServer.py
runtime/webinterface.js
runtime/__init__.py
runtime/ServicePublisher.py
runtime/typemapping.py 
runtime/loglevels.py 
Beremiz_service.py
version.py