Configure pytest's norecursedirs

This skips directories which don't contain unittests and also
virtualenvs (in folders named env*, which is already in gitignore)
as we don't want to execute any tests in there

setup.cfg

@@ -3,3 +3,6 @@ release = egg_info -RDb ''
 
 [wheel]
 universal = 1
+
+[pytest]
+norecursedirs = .* *.egg *.egg-info env* artwork docs