Bump pytest from 5.4.3 to 6.0.0 (#282)

Bumps [pytest](https://github.com/pytest-dev/pytest) from 5.4.3 to 6.0.0.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@5.4.3...6.0.0)

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

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

requirements-test.txt

@@ -7,7 +7,7 @@ git+git://github.com/scikit-learn-contrib/lightning.git@782c18c12961e509099ae84c
 
 # Testing tools
 flake8==3.8.3
-pytest==5.4.3
+pytest==6.0.0
 pytest-mock==3.2.0
 coveralls==2.1.1
 pytest-cov==2.10.0