stgit: python3.8, man pages, and completions
stgit: python3.8, man pages, and completions

Depend on python@3.8 instead of using the MacOS default python2.7. Upstream
StGit is dropping support for Python 2.x, so this needed to be done
eventually.

Also install the StGit man pages and completions for bash, fish, and zsh.

The test is updated to be slightly more robust.

Closes #51173.

Signed-off-by: Bo Anderson <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@boanderson.me>

nbari/homebrew-core,BrewTestBot/homebrew-core,rwhogg/homebrew-core,mvitz/homebrew-core,mahori/homebrew-core,mfikes/homebrew-core,jvillard/homebrew-core,nandub/homebrew-core,zyedidia/homebrew-core,lembacon/homebrew-core,jabenninghoff/homebrew-core,jvillard/homebrew-core,uyjulian/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,jabenninghoff/homebrew-core,edporras/homebrew-core,mbcoguno/homebrew-core,cblecker/homebrew-core,sjackman/homebrew-core,aabdnn/homebrew-core,sjackman/homebrew-core,mbcoguno/homebrew-core,JCount/homebrew-core,Linuxbrew/homebrew-core,adam-moss/homebrew-core,dsXLII/homebrew-core,Linuxbrew/homebrew-core,lembacon/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,spaam/homebrew-core,spaam/homebrew-core,zyedidia/homebrew-core,aabdnn/homebrew-core,lembacon/homebrew-core,mbcoguno/homebrew-core,filcab/homebrew-core,chrisfinazzo/homebrew-core,BrewTestBot/homebrew-core,uyjulian/homebrew-core,maxim-belkin/homebrew-core,makigumo/homebrew-core,nbari/homebrew-core,makigumo/homebrew-core,edporras/homebrew-core,mvitz/homebrew-core,jvillard/homebrew-core,mfikes/homebrew-core,nandub/homebrew-core,chrisfinazzo/homebrew-core,mvitz/homebrew-core,adam-moss/homebrew-core,Moisan/homebrew-core,Homebrew/homebrew-core,cblecker/homebrew-core,dsXLII/homebrew-core,stgraber/homebrew-core,Moisan/homebrew-core,stgraber/homebrew-core,mahori/homebrew-core,JCount/homebrew-core,lemzwerg/homebrew-core,lemzwerg/homebrew-core