Only set PYTHONPATH for Cython translation @ tox

It was overshadowing Fedora-injected variable in RPM builds.

Author: webknjaz

tox.ini

@@ -53,10 +53,9 @@ setenv =
   COV_CORE_CONFIG = {toxinidir}/.coveragerc
   COV_CORE_DATAFILE = {toxinidir}/.coverage
   COV_CORE_SOURCE =
-  PYTHONPATH = packaging/
   SEGFAULT_SIGNALS = all
 commands =
-  -{envpython} -m pep517_backend.cli translate-cython
+  env PYTHONPATH=packaging/ {envpython} -m pep517_backend.cli translate-cython
   {env:CATCHSEGV_BINARY:} {envpython} -m \
     pytest \
     {tty:--color=yes} \