Remove use of __file__

[jayvdb]

__file__ is an optional attribute of packages. c.f. indygreg/PyOxidizer#69 for why and alternatives.

spdx/config.py:_base_dir = os.path.dirname(__file__)

[nicoweidner]

The link to the promised solution results in a 404, do you have a different one?
https://pyoxidizer.readthedocs.io/en/latest/packaging_pitfalls.html#reliance-on-file

[jayvdb]

Seems to be now at https://pyoxidizer.readthedocs.io/en/stable/oxidized_importer_behavior_and_compliance.html?highlight=__file__#file-and-cached-module-attributes