Update pytest requirement from <4.6,>=3.6 to >=3.6,<4.7

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](pytest-dev/pytest@3.6.0...4.6.0)

test-requirements.txt

@@ -1,6 +1,6 @@
 coverage>=4.5.3,<4.6
 docutils==0.14
 flake8>=3.7,<3.8
-pytest>=3.6,<4.6
+pytest>=3.6,<4.7
 pytest-cov>=2.7.1,<2.8
 isort>=4.3.20,<4.4