get tags for rtd

[braingram]

Applies changes from https://github.com/spacetelescope/jwst/pull/8633

to allow removal of the stable branch (RTD will automatically generate one since we're semantically versioned).

I've already renamed the stable branch to "old_stable".

[zacharyburnett]

[zacharyburnett]

unfortunately it looks like the current stable docs didn't pick up the tags:

I reran the stable build to see if that fixes it

[braingram]

This sort of worked :-/

[jhunkeler]

Clearing the repo's dirty flag may be the only way to produce the expected version.

• https://docs.readthedocs.io/en/stable/build-customization.html#avoid-having-a-dirty-git-index
• https://github.com/spacetelescope/jwst/pull/8644