Bump pytest-cov from 2.6.1 to 2.11.0

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.6.1 to 2.11.0.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.6.1...v2.11.0)

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

requirements-dev.txt

@@ -2,6 +2,6 @@
 
 coverage==4.5.3
 pytest==4.4.0
-pytest-cov==2.6.1
+pytest-cov==2.11.0
 responses==0.10.6
 coveralls==1.7.0