Revert "Pin pytest whilst pytest-rerunfailures is incompatible with pytest 6.1"

This reverts commit fbf710ea5d.

tools/requirements/tests.txt

@@ -5,7 +5,7 @@ enum34; python_version < '3.4'
 freezegun
 mock
 pretend
-pytest<6.1.0 # Until https://github.com/pytest-dev/pytest-rerunfailures/issues/128 is fixed
+pytest
 pytest-cov
 pytest-rerunfailures
 pytest-timeout