temp/gtk+3.0: upgrade to 9999_git20210108

Fix about dialogs not being adaptive and all other changes by Purism, which are the reason why we use the fork in the first place. The patches didn't get applied in the previous version we had packaged in postmarketOS, because the patches are in debian/patches now.

Remove check-version.py, it's in the source tree now.

---

I've verified that this works on the pinephone.

Close #951 (closed)