License change: snakemake 7.20.0 is MIT AND Unlicense

[Ben Beasley]

The entire project remains (SPDX) MIT, except:

- versioneer.py is Unlicense (but is not packaged in the binary RPMs)

- snakemake/_version.py says:

     This file is released into the public domain.

which would be LicenseRef-Fedora-Public-Domain, except that the comments 
in versioneer.py make it clear that Unlicense is intended for the 
generated files as well. The License field now reflects this.

Older versions of versioneer.py had similar language, but with CC0-1.0 
instead of Unlicense.