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

BEREMIZ_OBJS = $(BEREMIZSRC:.c=.o)

all: warn some_binary
	@echo "*** all done ***"

warn:
	@echo "*** Sample Makefile, does nothing ***"

some_binary: $(BEREMIZ_OBJS)
	@echo "* Would link $^ -> $@"

%.o: %.c
	@echo "* Would compile $< -> $@"