Use compatible coverage version

GitHub PyPy version 7.3.7 needs 6.3.2 at the moment
as 6.3.3 is only available for newer PyPy version 7.3.8

requirements-test.txt

@@ -1,5 +1,5 @@
 -e .
-coverage==6.3.3
+coverage==6.3.2
 pytest==7.1.2
 pytest-cov==3.0.0
 pytest-django==4.5.2