Bump pytest from 4.6.9 to 5.4.1

Bumps [pytest](https://github.com/pytest-dev/pytest) from 4.6.9 to 5.4.1.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@4.6.9...5.4.1)

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

requirements/dev.txt

@@ -1,4 +1,4 @@
 coverage==5.0.4
 mock==3.0.5 # last version compatible with python2.7
-pytest==4.6.9 # last version compatible with python2.7
+pytest==5.4.1 # last version compatible with python2.7
 pytest-mock == 2.0.0