[TESTING] Possible fix for differing results in CI (- WIP #46 & #49 -)

### ChangeLog:

Changes in file Makefile:
 test-style: cleanup must_have_flake

Makefile

@@ -177,11 +177,11 @@ test-style: cleanup must_have_flake
 	$(QUIET)$(ECHO) "$@: Done."
 
 must_have_flake:
-	$(QUIET)runner=`python3 -m pip freeze --all | grep --count -oF flake` ; \
+	$(QUIET)runner=`$(PYTHON) -m pip freeze --all | grep --count -oF flake` ; \
 	if test $$runner -le 0 ; then $(ECHO) "No Linter found for test." ; exit 126 ; fi
 
 must_have_pytest:
-	$(QUIET)runner=`python3 -m pip freeze --all | grep --count -oF pytest` ; \
+	$(QUIET)runner=`$(PYTHON) -m pip freeze --all | grep --count -oF pytest` ; \
 	if test $$runner -le 0 ; then $(ECHO) "No python framework (pytest) found for test." ; exit 126 ; fi
 
 cleanup: