New flake8 version raises new linting errors

[armintaenzertng]

With flake8 version 6.1.0, new linting errors appeared in the CI:

tests/spdx/parser/all_formats/test_parse_from_file.py:39:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/spdx/parser/all_formats/test_parse_from_file.py:51:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/spdx/parser/jsonlikedict/test_dict_parsing_functions.py:37:12: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
tests/spdx/parser/jsonlikedict/test_dict_parsing_functions.py:46:12: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`

These need to be fixed so that the CI comes out green again and other PRs can be merged.

[armintaenzertng]

fixed by #743