setup: bump py.test version

Note: latest pytest versions has an undeclared dependency / incompatibility
with pytest-benchmark, which thus has to be bumped at the same time:

INTERNALERROR> Traceback (most recent call last):
INTERNALERROR> File "data/env/lib/python2.7/site-packages/_pytest/main.py", line 202, in wrap_session
INTERNALERROR> config._do_configure()
INTERNALERROR> File "data/env/lib/python2.7/site-packages/_pytest/config/__init__.py", line 671, in _do_configure
INTERNALERROR> self.hook.pytest_configure.call_historic(kwargs=dict(config=self))
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pluggy/hooks.py", line 311, in call_historic
INTERNALERROR> res = self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pluggy/manager.py", line 87, in _hookexec
INTERNALERROR> return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pluggy/manager.py", line 81, in <lambda>
INTERNALERROR> firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pluggy/callers.py", line 208, in _multicall
INTERNALERROR> return outcome.get_result()
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pluggy/callers.py", line 81, in get_result
INTERNALERROR> _reraise(*ex) # noqa
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR> res = hook_impl.function(*args)
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pytest_benchmark/plugin.py", line 427, in pytest_configure
INTERNALERROR> bs = config._benchmarksession = BenchmarkSession(config)
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pytest_benchmark/session.py", line 31, in __init__
INTERNALERROR> self.logger = Logger(self.verbose, config)
INTERNALERROR> File "data/env/lib/python2.7/site-packages/pytest_benchmark/logger.py", line 15, in __init__
INTERNALERROR> self.pytest_warn = config.warn
INTERNALERROR> AttributeError: 'Config' object has no attribute 'warn'


The new py.test will show deprecation warnings from other libraries and how we use them:

kallithea/tests/__init__.py:28
kallithea/tests/__init__.py:28: PytestAssertRewriteWarning: Module already imported so cannot be rewritten: kallithea.tests
pytest.register_assert_rewrite('kallithea.tests')

data/env/lib/python2.7/site-packages/pkg_resources/__init__.py:1145
kallithea/tests/api/test_api_git.py::TestGitApi::test_api_wrong_key
data/env/lib/python2.7/site-packages/pkg_resources/__init__.py:1145: DeprecationWarning: Use of .. or absolute path in a resource path is not allowed and will raise exceptions in a future release.
self, resource_name

<string>:2
<string>:2: SADeprecationWarning: Mapper.order_by is deprecated.Use Query.order_by() in order to affect the ordering of ORM result sets.

data/env/lib/python2.7/site-packages/tg/configuration/app_config.py:473
data/env/lib/python2.7/site-packages/tg/configuration/app_config.py:473: DeprecationWarning: direct usage of error tracing options has been deprecated, please specify them as trace_errors.option_name instad of directly setting option_name. EXAMPLE: trace_errors.error_email
"setting option_name. EXAMPLE: trace_errors.error_email", DeprecationWarning)

data/env/lib/python2.7/site-packages/tg/wsgiapp.py:68
data/env/lib/python2.7/site-packages/tg/wsgiapp.py:68: DeprecationWarning: Session options should start with session. instead of baker.session.
app_wrapper = wrapper(self.wrapped_dispatch, self.config)

...
kallithea/model/validators.py:279: DeprecationWarning: validate_python is deprecated; use _validate_python instead
class _validator(formencode.validators.FancyValidator):

...
kallithea/model/validators.py:793: DeprecationWarning: _to_python is deprecated; use _convert_to_python instead
class _validator(formencode.validators.FancyValidator):

...

kallithea/tests/other/test_doctest.py::test_doctests[kallithea.lib.markup_renderer]
data/env/lib/python2.7/site-packages/markdown/__init__.py:259: DeprecationWarning: Using short names for Markdown's builtin extensions is deprecated. Use the full path to the extension with Python's dot notation (eg: "markdown.extensions.codehilite" instead of "codehilite"). The current behavior will raise an error in version 2.7. See the Release Notes for Python-Markdown version 2.6 for more info.
DeprecationWarning)

kallithea/tests/other/test_doctest.py::test_doctests[kallithea.lib.markup_renderer]
data/env/lib/python2.7/site-packages/markdown/__init__.py:259: DeprecationWarning: Using short names for Markdown's builtin extensions is deprecated. Use the full path to the extension with Python's dot notation (eg: "markdown.extensions.extra" instead of "extra"). The current behavior will raise an error in version 2.7. See the Release Notes for Python-Markdown version 2.6 for more info.
DeprecationWarning)