Bump pytest-mock from 3.0.0 to 3.1.1 (#244)

Bumps [pytest-mock](https://github.com/pytest-dev/pytest-mock) from 3.0.0 to 3.1.1.
- [Release notes](https://github.com/pytest-dev/pytest-mock/releases)
- [Changelog](https://github.com/pytest-dev/pytest-mock/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-mock@v3.0.0...v3.1.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements-test.txt

@@ -8,7 +8,7 @@ git+git://github.com/scikit-learn-contrib/lightning.git@b96f9c674968496e85407816
 # Testing tools
 flake8==3.7.9
 pytest==5.4.3
-pytest-mock==3.0.0
+pytest-mock==3.1.1
 coveralls==2.0.0
 pytest-cov==2.10.0