Revisit pytest config

- add testpaths setting
- tox: remove testpaths from posargs default
- s/py.test/pytest/

tox.ini

@@ -25,7 +25,7 @@ setenv =
     env36: JEDI_TEST_ENVIRONMENT=36
     env37: JEDI_TEST_ENVIRONMENT=37
 commands =
-    py.test {posargs:jedi test}
+    pytest {posargs}
 [testenv:py27]
 deps =
 # for testing the typing module
@@ -56,7 +56,7 @@ deps =
     numpydoc
     {[testenv]deps}
 commands =
-    coverage run --source jedi -m py.test
+    coverage run --source jedi -m pytest
     coverage report
 [testenv:sith]
 commands =