Update usage of stregions with Astropy regions

[stscijgbot-hstdp]

Issue HLA-1156 was created on JIRA by Steve Goldman:

Readthedocs has updated to using python 3.12 and because of this, a long-deprecated dependency "stregions" is no longer able to build with a pip wheel. It is only used in two files to read/parse ds9 region files and so it should be feasible to replace these with astropy regions. 

 

This is required to return to building the readthedocs documentation. 

[stscijgbot-hstdp]

Comment by Steve Goldman on JIRA:

duplicated by #⁠1162