Expand `Mypy` ruling :crown:

[FirelightFlagboy]

• [ ] Enable mypy on protocol.invite: #3255
• [x] Enable mypy on rust pyi: #3239
• [ ] Enable mypy on core
  • [ ] #3319
  • [ ] #3318
  • [ ] Enable mypy on core.gui
  • [ ] Enable mypy on core.invite
  • [ ] #3303
  • [ ] Enable mypy on core.pki
  • [ ] #3284
• [ ] Enable mypy on backend
  • [ ] #3289
  • [ ] #3293
  • [ ] #3296
  • [ ] #3322
• [ ] Enable mypy on api
• [ ] Enable mypy on serde
• [ ] Enable mypy on stubs.mashmallow
• [ ] Enable mypy on test_utils
• [ ] Enable mypy on . (on the python files in parsec/)