warnings.warning() does not exist

[jeswan]

Bug Description:
When using stempeg as part of musdb, I encountered the following error:

        stem_durations = np.array([t.shape[0] for t in stems])
        if not (stem_durations == stem_durations[0]).all():
>           warnings.warning("Stems differ in length and were shortend")
E           AttributeError: module 'warnings' has no attribute 'warning'

/usr/local/lib/python3.9/site-packages/stempeg/read.py:299: AttributeError

warning() does not exist after checking the warnings package.

Suggested Solution:
warnings.warning() -> warnings.warn() since warn() exists.

[faroit]

oops. good catch. can fix that next week - you can also provide a PR if you like