Replace mypy-protobuf with protobuf pyi_out

I noticed that pyi_out is a lot cleaner and more readable that mypy-protobuf. Since pyi generation is part of protobuf it is probably prefered instead of unofficial mypy_protobuf.

And there is also one interesting sideeffect that enums constants are not compatible between themselves exposing couple of bugs in our code.

For example:

enum EnumA {
  A_A = 0;
  A_B = 1;
}

enum EnumB {
  B_A = 0;
  B_B = 1;
}

message Msg {
  EnumA name = 1;
}

mypy is not bothered that name can also be used with B_A and B_B. But it works with pyi_out generated code.

Start by locating the repository configuration or scripts that invoke mypy-protobuf and inspect how generated Python stubs are used. Compare the current output with protobuf's pyi_out behavior, then verify that the replacement preserves the expected stubs and prevents incompatible enum constants from being accepted.