Update pytest requirement from ~=3.8.0 to ~=3.8.1

Updates the requirements on [pytest](https://github.com/pytest-dev/pytest) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/commits/3.8.1)

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

test-requirements.txt

@@ -1,3 +1,3 @@
 nosexcover~=1.0.11
 pylint~=2.1.1
-pytest~=3.8.0
+pytest~=3.8.1