Bump pytest from 3.5.1 to 3.6.4

Bumps [pytest](https://github.com/pytest-dev/pytest) from 3.5.1 to 3.6.4.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@3.5.1...3.6.4)

Signed-off-by: dependabot[bot] <support@dependabot.com>

dev-requirements.txt

@@ -4,7 +4,7 @@ aioresponses==0.3.0
 cchardet==2.1.1
 flake8==3.5.0
 mock==2.0.0
-pytest==3.5.1
+pytest==3.6.4
 pytest-aiohttp==0.1.3
 pytest-asyncio==0.8.0
 pytest-cov==2.5.1