Add LICENSE file to release tarball

[opoznyak]

Steps to reproduce

1. Download release tarball from here -> https://pypi.org/project/pydub/#files
2. Unpack and check for any LICENSE file there

Expected behavior

MIT LICENSE should be present there.

Actual behavior

No LICENSE is there

Your System configuration

• Python version: 3.8.2
• Pydub version: 0.24.1
• ffmpeg or avlib?: ffmpeg
• ffmpeg/avlib version: 4.2.2

Is there an audio file you can include to help us reproduce?

None

[kynan]

@jiaaro any chance this can be fixed? This is currently a blocker for conda packaging pydub.