Update pytest requirement from <4.6,>=3.7.2 to >=3.7.2,<4.7 in /src

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@3.7.2...4.6.0)

src/requirements-test.txt

@@ -1,2 +1,2 @@
-pytest>=3.7.2,<4.6
+pytest>=3.7.2,<4.7
 pytest-cov>=2.5.1,<2.8