Replace mypy-protobuf with protobuf pyi_out

[matejsp]

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.

[Avasam]

Related: https://github.com/nipunn1313/mypy-protobuf/issues/628 and https://github.com/nipunn1313/mypy-protobuf/issues/368#issuecomment-1223884445

It would be interesting to do a side-by-side comparison.

[aidandj]

https://github.com/protocolbuffers/protobuf/issues/23670

This is at least one difference.

There are also differences in enum value types that are incompatible across the 2 implementations. So it would be a breaking change.

[aidandj]

There is now a somewhat up to date comparison here