Bump pytest-cov from 2.7.1 to 2.8.1

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.7.1 to 2.8.1.
- [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@v2.7.1...v2.8.1)

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

requirements/local.txt

@@ -10,7 +10,7 @@ psycopg2-binary==2.7.5  # https://github.com/psycopg/psycopg2
 mypy==0.641  # https://github.com/python/mypy
 pytest==3.9.3  # https://github.com/pytest-dev/pytest
 pytest-sugar==0.9.1  # https://github.com/Frozenball/pytest-sugar
-pytest-cov==2.7.1
+pytest-cov==2.8.1
 requests==2.22.0
 iiif-prezi==0.2.9 # https://github.com/iiif-prezi/iiif-prezi
 httpretty==0.9.6 # https://pypi.org/project/httpretty/