warnings.warning() does not exist

**Bug Description:**
When using `stempeg` as part of [`musdb`](https://github.com/sigsep/sigsep-mus-db), I encountered the following error:

```python
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.