Revert unexpected API changes in 30->31
The changes in parser.py at https://gitlab.com/inorton/junit2html/-/merge_requests/35/diffs#6176e24109ad308a2b35681a962f03204903db9a_17_23 where the module level constants for `PASSED` and other states are, moved them to class members. These should be moved back to module-level
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