setup: don't pass --strict to pytest

pytest 8 removed that option, but the replacement --strict-markers
wasn't added until 4.5.0, and we're using 3.10.1 on py2.

setup.cfg

@@ -21,7 +21,6 @@ python_files =
     tests.py
 addopts =
     -rxEfsw
-    --strict
     --doctest-modules
     --doctest-glob=\*.rst
     --tb=short