Update hi.py

[dvstark]

Fixes # Adds HI-MaNGA VAC functionality for DR17 by adding routine to select best spectrum between GBT vs. ALFALFA observations where necessary. Updates "version" entry with v2_0_1 for HI-MaNGA

This pull request:

• Has a title that summarises what is changing.
• Updates the documentation accordingly.
• Has unit tests & code coverage is not adversely affected (within reason).
• Works with Python 2.7 and 3.6 (and ideally with Python 3.7).
• Updates the CHANGELOG.
• Removes more lines of code than it adds.
• If relevant, adds a new entry to the What's new? page.

[havok2063]

Overall I think this is good. Once merged, I'll do some final checks and tests.