Unpin pytest and its plugins

This works around the incompatibility of pytest-xdist 2.0.0
with older pytest: pytest-dev/pytest-xdist#580

tools/requirements/tests.txt

@@ -4,16 +4,14 @@ enum34; python_version < '3.4'
 freezegun
 mock
 pretend
-# pytest 5.x only supports python 3.5+
-pytest<5.0.0
+pytest
 pytest-cov
-# Prevent installing 9.0 which has install_requires "pytest >= 5.0".
-pytest-rerunfailures<9.0
+pytest-rerunfailures
 pytest-timeout
 pytest-xdist
 pyyaml
-setuptools>=39.2.0  # Needed for `setuptools.wheel.Wheel` support.
 scripttest
+setuptools>=39.2.0  # Needed for `setuptools.wheel.Wheel` support.
 https://github.com/pypa/virtualenv/archive/legacy.zip#egg=virtualenv
 werkzeug==0.16.0
 wheel