Bump pytest-mock from 3.4.0 to 3.5.1

Bumps [pytest-mock](https://github.com/pytest-dev/pytest-mock) from 3.4.0 to 3.5.1.
- [Release notes](https://github.com/pytest-dev/pytest-mock/releases)
- [Changelog](https://github.com/pytest-dev/pytest-mock/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest-mock/compare/v3.4.0...v3.5.1)

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

requirements-dev.txt

@@ -1,11 +1,11 @@
 -e .
 flake8==3.8.4
 async-timeout==3.0.1
-pytest==6.1.2
+pytest==6.2.1
 pytest-cov==2.10.1
-pytest-mock==3.3.1
-coverage==5.3
-sphinx==3.3.1
+pytest-mock==3.5.1
+coverage==5.3.1
+sphinx==3.4.1
 pep257==0.7.0
 aiohttp-session==2.9.0
 aiopg[sa]==1.1.0