setup.py: Properly add _sdl2_data_files to data_files

[cclauss]

Currently, this code is ALWAYS adding an empty list instead of the data files that were found by glob.glob().

It adds the empty list before filling the list!

[cclauss]

TIL:

>>> big = []
>>> little = []
>>> big.append(("little", little))
>>> big
[('little', [])]
>>> for c in "abc":
...     little.append(c)
...
>>> little
['a', 'b', 'c']
>>> big
[('little', ['a', 'b', 'c'])]

[cclauss]

Once pre-commit is merged, we can add auto-walrus if you would like...

• 2772

% auto-walrus **/*.py

Rewriting buildconfig/msysio.py
Rewriting src_py/sprite.py
Rewriting src_py/sysfont.py
Rewriting test/controller_test.py
Rewriting test/mask_test.py
Rewriting test/mixer_test.py
Rewriting test/test_utils/test_runner.py