Do not use pytest 7.0.0 which is buggy

Reference: pytest-dev/pytest#9608
Signed-off-by: Philippe Ombredanne <[email protected]>

Author: pombredanne

Diff for: setup.cfg

@@ -37,8 +37,7 @@ where=src
 
 [options.extras_require]
 testing =
-    # upstream
-    pytest >= 6
+    pytest >= 6, != 7.0.0
     pytest-xdist >= 2
 docs=
     Sphinx>=3.3.1