Improve support for messages/enums with python reserved keyword names

[nipunn1313]

See #221 for discussion

message None {
    string field = 1;
}
message M {
    None none = 1;
}

Currently, mypy-protobuf does not generate stubs for None and generates a field accessor

class M:
  @property
  def none() -> Any

We can improve this to

class None_:
    ...

class M:
    @property
    def none() -> None_

This will allow accessors to such messages improved typing