[Bug] Separate.separate returns None is an invalid type annotation

[mxkrn]

• [x] I didn't find a similar issue already open.
• [x] I read the documentation (README AND Wiki)
• [x] I have installed FFMpeg
• [x] My problem is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)

Description

The type annotation for Separator.separate indicates that it should return None, this is invalid since both _separate_librosa and _separate_tensorflow return a Dict.

Step to reproduce

Just have a look at the code at https://github.com/deezer/spleeter/blob/4aba6011d60f234ed10ab4c66e863fb26834f7bd/spleeter/separator.py#L305

Additional Notes

This was flagged by mypy==0.910 when using the Separator class in a python script.