tox: Update pytest to 2.8.6.

- allow for double nodeids in junitxml,
  this was a regression failing plugins combinations like pytest-pep8 +
  pytest-flakes

- Workaround for exception that occurs in pyreadline when using ``--pdb`` with
  standard I/O capture enabled.

- Better error message in case the target of a ``monkeypatch`` call raises an
  ``ImportError``.

- monkeypatch calls (setattr, setenv, etc.) are now O(1).

- captured stdout and stderr are now properly displayed before entering pdb
  when ``--pdb`` is used instead of being thrown away.

- pytest warnings emitted during ``pytest_terminal_summary`` are now properly
  displayed.

- fixed internal UnicodeDecodeError when doctests contain unicode.

- Add captured stdout to jUnit XML report on setup error.

tox.ini

@@ -26,7 +26,7 @@ deps =
     parse==1.6.6
     parse-type==0.3.4
     py==1.4.31
-    pytest==2.8.5
+    pytest==2.8.6
     pytest-bdd==2.16.0
     pytest-catchlog==1.2.1
     pytest-cov==2.2.0