Update pytest requirement from ==5.0.* to ==5.1.* (#391)

Updates the requirements on [pytest](https://github.com/pytest-dev/pytest) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@5.0.0...5.1.0)

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

requirements/test.txt

@@ -1,4 +1,4 @@
-pytest==5.0.*
+pytest==5.1.*
 pytest-cov==2.7.*
 pytest-django==3.5.*
 pytest-pythonpath==0.7.*