Update pytest-cov requirement from ~=2.6 to ~=2.8 (appium#489)

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [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@v2.6.0...v2.8.1)

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

Author: dependabot-preview[bot]

Pipfile

@@ -12,7 +12,7 @@ selenium = "~=3.141"
 autopep8 = "~=1.4"
 
 pytest = "~=4.0"
-pytest-cov = "~=2.6"
+pytest-cov = "~=2.8"
 
 tox = "~=3.6"
 tox-travis = "~=0.11"