devel/py-python-magic: Update to 0.4.15

• Add USE_GITHUB comment (missing test data files)

There are a number of test failures that are a consequence of slight differences
between platform magic implementations and/or databases. Two have been reported
upstream [1][2] and will be resolved in a subsequent release.

[1] https://github.com/ahupp/python-magic/issues/173
[2] https://github.com/ahupp/python-magic/issues/96

PR: 234600
Submitted by: Gilbert Morgan <gmm tutanota com>