Motivation for this change

This won’t get picked up by r-ryantm.

Changes look okay:

3.36.1 - 2020-05-06

• tests: Fix failing tests with pytest 5.4.0+
• Gtk: Add override to make sure both TreeModelSort.new_with_model and
  TreeModel.sort_new_with_model exist independend of the gtk version
• Gtk.Template: Fix initialisation order errors with Widgets getting created from C
  (potentially through other templates) :issue:257 :issue:386 :issue:341 :mr:140 (:user:Jean Felder <jfelder>)
• Gtk.Template: Fix errors when calling init_template() multiple times :mr:140 (:user:Jean Felder <jfelder>)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.