tox: Update pytest to 2.7.3.

Changelog:

- Allow 'dev', 'rc', or other non-integer version strings in `importorskip`.
  Thanks to Eric Hunsberger for the PR.

- fix issue856: consider --color parameter in all outputs (for example
  --fixtures). Thanks Barney Gale for the report and Bruno Oliveira for the PR.

- fix issue855: passing str objects as `plugins` argument to pytest.main
  is now interpreted as a module name to be imported and registered as a
  plugin, instead of silently having no effect.
  Thanks xmo-odoo for the report and Bruno Oliveira for the PR.

- fix issue744: fix for ast.Call changes in Python 3.5+.  Thanks
  Guido van Rossum, Matthias Bussonnier, Stefan Zimmermann and
  Thomas Kluyver.

- fix issue842: applying markers in classes no longer propagate this markers
  to superclasses which also have markers.
  Thanks xmo-odoo for the report and Bruno Oliveira for the PR.

- preserve warning functions after call to pytest.deprecated_call. Thanks
  Pieter Mulder for PR.

- fix issue854: autouse yield_fixtures defined as class members of
  unittest.TestCase subclasses now work as expected.
  Thannks xmo-odoo for the report and Bruno Oliveira for the PR.

- fix issue833: --fixtures now shows all fixtures of collected test files, instead of just the
  fixtures declared on the first one.
  Thanks Florian Bruhin for reporting and Bruno Oliveira for the PR.

- fix issue863: skipped tests now report the correct reason when a skip/xfail
  condition is met when using multiple markers.
  Thanks Raphael Pierzina for reporting and Bruno Oliveira for the PR.

- optimized tmpdir fixture initialization, which should make test sessions
  faster (specially when using pytest-xdist). The only visible effect
  is that now pytest uses a subdirectory in the $TEMP directory for all
  directories created by this fixture (defaults to $TEMP/pytest-$USER).
  Thanks Bruno Oliveira for the PR.

tox.ini

@@ -16,7 +16,7 @@ passenv = PYTHON DISPLAY XAUTHORITY HOME USERNAME USER CI
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.2
+    pytest==2.7.3
     pytest-capturelog==0.7
     pytest-qt==1.6.0
     pytest-mock==0.7.0
@@ -106,7 +106,7 @@ passenv =
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.2
+    pytest==2.7.3
     pyflakes==0.9.2
     pytest-flakes==1.0.0
 commands =
@@ -119,7 +119,7 @@ passenv =
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.2
+    pytest==2.7.3
     pep8==1.6.2
     pytest-pep8==1.0.6
 commands =
@@ -132,7 +132,7 @@ passenv =
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.30
-    pytest==2.7.2
+    pytest==2.7.3
     mccabe==0.3.1
     pytest-mccabe==0.1
 commands =