Add GHA workflow to automatically update pre-commit dependencies

[justvanrossum]

dependabot doesn't support pre-commit yet, so we need another solution.

Fabio has an example workflow file that would do this on a monthly basis, which is perfect.

[fabiocaccamo]

• [x] fontra
• [x] fontra-rcjk
• [x] fontra-glyphs
• [x] fontra-compile
• [x] fontra-pak
• [x] django-robo-cjk

[justvanrossum]

I'm giving up on this approach: isn't really working well enough, so I'll take it on me to do regular pre-commit updates manually every once in a while.