DOC: Specify py2k-compatible version in py3k statement

[DWesl]

The python>=3.6 requirement at the top of the file dates to early 2020, before v0.1 of MONETIO. Before this, MONET and MONETIO were one package. It looks like the last commit in MONET 2.1.5 merges in the work to split MONET and MONETIO, so someone wanting MONETIO functionality from MONET would need a MONET version prior to that.

[DWesl]

I wonder if we could include your reference to v2.1.5 as a footnote referenced here

https://github.com/noaa-oar-arl/monet/blob/987a1fe77fc8297edadb42710c3d7fc1ab20b616/docs/index.rst?plain=1#L42

to show that technically this was associated with v2.1.5 / noaa-oar-arl/monet#77, though v2.2.0 was the first GitHub Release. I can do this in MONET repo.

Like this?

EDIT: Wrong repo. Should I leave it anyway?

[zmoon]

https://monetio--212.org.readthedocs.build/en/212/index.html

[zmoon]

Thanks @DWesl