Revert "Cap pytest to <6.0.0 to fix pytest-html"

This reverts commit 9946a85183.

The problem was fixed with pytest 6.0.1

  https://docs.pytest.org/en/stable/changelog.html#pytest-6-0-1-2020-07-30

Change-Id: I580a96e6eb65c272d9306af118b8f27ae8cff8c7

test-requirements.txt

@@ -9,12 +9,10 @@ openstacksdk
 zuul-sphinx>=0.2.3
 testtools
 mock
-# pytest 6.0.0 breaks pytest-html
-pytest<6.0.0
-pytest-html # MPL-2.0
 # testinfra 1.17.0 has a broken wheel that won't install under python3
 # testinfra 3.0.0 switched the ansible backend, but broke ipv6 handling
 testinfra!=1.17.0,!=3.0.0
+pytest-html # MPL-2.0
 # testinfra's ansible backend actually requires paramiko, but it's not
 # specified.  After
 #  https://github.com/philpep/testinfra/pull/454