tox: Update to pytest 2.9.1

- Improve error message when a plugin fails to load.
- pytest.fail with non-ascii characters raises an internal pytest error.
- junit parses report.nodeid incorrectly, when params IDs contain ::.
- SyntaxErrors containing non-ascii lines at the point of failure generated an
  internal py.test error.
- When passing in a bytestring regex pattern to parameterize attempt to decode
  it as utf-8 ignoring errors.
- parametrized test nodes cannot be specified to run on the command line.

tox.ini

@@ -26,7 +26,7 @@ deps =
     parse==1.6.6
     parse-type==0.3.4
     py==1.4.31
-    pytest==2.9.0
+    pytest==2.9.1
     pytest-bdd==2.16.0
     pytest-catchlog==1.2.2
     pytest-cov==2.2.1