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

image: skvorl/beremiz-requirements

pipelines:
  default:
    - parallel:
        - step:
            name: Codestyle checks
            script:
              - ln -s /CanFestival-3 $BITBUCKET_CLONE_DIR/../CanFestival-3
              - /usr/bin/python --version
              - ./tests/tools/check_source.sh
        - step:
            name: Application tests
            max-time: 10
            script:
              - ln -s /CanFestival-3 $BITBUCKET_CLONE_DIR/../CanFestival-3
              - ./tests/tools/run_python_tests.sh