Update pytest requirement from <6 to <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@1.0.0b3...6.0.0)

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

Author: dependabot-preview[bot]

test_requirements.txt

@@ -1,4 +1,4 @@
-pytest<6
+pytest<7
 hypothesis<5.23.4
 coverage==5.2.1
 attrs==19.3.0