[stable-2.8] Update constraints for coverage on Python 3.8 (#59733)

This fixes the unit tests hanging when run with Python 3.8 with coverage enabled
(cherry picked from commit ed4a729fd6)

Co-authored-by: Sam Doran <sdoran@redhat.com>

test/runner/requirements/constraints.txt

@@ -1,4 +1,5 @@
-coverage >= 4.2, != 4.3.2 # features in 4.2+ required, avoid known bug in 4.3.2 on python 2.6
+coverage >= 4.2, != 4.3.2 ; python_version <= '3.7' # features in 4.2+ required, avoid known bug in 4.3.2 on python 2.6
+coverage >= 4.5.4 ; python_version > '3.7' # coverage had a bug in < 4.5.4 that would cause unit tests to hang in Python 3.8
 cryptography < 2.2 ; python_version < '2.7' # cryptography 2.2 drops support for python 2.6
 deepdiff < 4.0.0 ; python_version < '3' # deepdiff 4.0.0 and later require python 3
 urllib3 < 1.24 ; python_version < '2.7' # urllib3 1.24 and later require python 2.7 or later