Upgrade pytest, and list it as a test-only dependency

Change-Id: Id1cbfb88d3034fc01f302af53bbd689afe54ffa5

requirements.txt

@@ -6,7 +6,6 @@ numpy>=1.16.1,<2
 pandas==0.24.2
 pbr>=5.4.4,<6
 Pygments>=2.4.2,<3
-pytest>=4.0.0,<5
 scipy>=1.3.0,<2
 Sphinx>=2.4.4,<3
 sphinxcontrib-bibtex>=0.4.2,<1

tox.ini

@@ -5,6 +5,7 @@ skipsdist = True
 [testenv]
 deps =
 	-r{toxinidir}/requirements.txt
+	pytest>=5.0.0,<6
 	cover: pytest-cov
 changedir = {toxinidir}
 usedevelop = True