switch test runner from nose to pytest

tests/test_ci.py

@@ -0,0 +1,9 @@
+def test_ci_placeholder():
+    # This empty test is used within the CI to
+    # setup the tox venv without running the test suite
+    # if we simply skip all test with pytest -k=wrong_pattern
+    # pytest command would return with exit_code=5 (i.e "no tests run")
+    # making travis fail
+    # this empty test is the recommended way to handle this
+    # as described in https://github.com/pytest-dev/pytest/issues/2393
+    pass