sourcehut: add override for python3 flask dependency

[christoph-heiss]

Description of changes

flask was recently updated to it's latest version 3.0.1 (a24d9a6a54fb), which in turn breaks the override for flask-sqlalchemy 2.5.1, as this requires flask 2.x.

At the end, this then breaks (building) sourcehut.

As flask 2.x also requires werkzeug 2.x, a override for that package is also needed.

The testsuite for sourcehut runs through again without hiccup with this applied.

Things done

• Built on platform(s)
  • [x] x86_64-linux
  • [ ] aarch64-linux
  • [ ] x86_64-darwin
  • [ ] aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • [ ] sandbox = relaxed
  • [ ] sandbox = true
• [x] Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• [x] Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• [x] Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • [ ] (Package updates) Added a release notes entry if the change is major or breaking
  • [ ] (Module updates) Added a release notes entry if the change is significant
  • [ ] (Module addition) Added a release notes entry if adding a new NixOS module
• [x] Fits CONTRIBUTING.md.

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