PYI generator: print values as fields of enum

[dbeinder]

When generating PYI for python, enums don't have any fields and your IDE shows errors when referring to a value of an enum using MyEnum.Foo

Example:

syntax = "proto3";
enum MyEnum {
	Abc = 0;
	Foo = 1;
	Bar = 2;
};

$> protoc --pyi_out=. myenum.proto

Current PYI Output:

from google.protobuf.internal import enum_type_wrapper as _enum_type_wrapper
from google.protobuf import descriptor as _descriptor
from typing import ClassVar as _ClassVar

DESCRIPTOR: _descriptor.FileDescriptor

class MyEnum(int, metaclass=_enum_type_wrapper.EnumTypeWrapper):
    __slots__ = []
Abc: MyEnum
Foo: MyEnum
Bar: MyEnum

With my changes:

from google.protobuf.internal import enum_type_wrapper as _enum_type_wrapper
from google.protobuf import descriptor as _descriptor
from typing import ClassVar as _ClassVar

DESCRIPTOR: _descriptor.FileDescriptor

class MyEnum(int, metaclass=_enum_type_wrapper.EnumTypeWrapper):
    __slots__ = []
    Abc: MyEnum
    Foo: MyEnum
    Bar: MyEnum

Abc: MyEnum
Foo: MyEnum
Bar: MyEnum

It's a bit ugly to have all enum values twice in the interface file, but it is an accurate representation of the generated Python interface - all the values are always in the enum class as well as the parent block. The change also works with enums nested inside of classes.

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[anandolee]

Thanks for the PR We have added some tests for stub, I may need to port this change into our internal first and apply to the tests.

[anandolee]

An internal change is under review. Close this one for clean up