Bump pytest-cov from 2.3.0 to 2.7.1

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.3.0 to 2.7.1.
- [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@v2.3.0...v2.7.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -1,5 +1,5 @@
 pytest==2.9.2
-pytest-cov==2.3.0
+pytest-cov==2.7.1
 pytest-runner==2.9
 responses==0.8.1
 twine==1.11.0