setup: depend on pathlib only on old python versions

since 3.4 this is part of standard library - and having this installed
on python3.7 cause some issues (for example when running pylint)

setup.py

@@ -32,7 +32,7 @@ setup(name=name,
       extras_require={
           'test': [
               'mock',
-              'pathlib',
+              'pathlib; python_version<"3.4"',
           ],
       },
       entry_points={