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

     1

     2

                  <xsd:element name="Xenomai">

     3

                    <xsd:complexType>

     4

                      %(toolchain_gcc)s

     5

                      <xsd:attribute name="XenoConfig" type="xsd:string" use="optional" default="xeno-config"/>

     6

                    </xsd:complexType>

     7

                  </xsd:element>