fix (protoc-gen-twirpy): service url is not compatible with golang server

[khevse]

twirp does't use original service name from protobuf:
https://github.com/twitchtv/twirp/blob/master/protoc-gen-twirp_python/main.go#L142
But it's in a private package. So I created a compatible version.

[ofpiyush]

Thanks for the PR @khevse! Sorry for the late response, I'd accidentally unwatched this repo.

I did not know that they change the names in the URL.

After learning about this, I checked their go generator code as that one seems to be better maintained.

They apparently do it for methods as well.

I'll try to figure out their stance on this and see if we can get this added in the spec for clarity and maybe have the CamelCase method part of a public API.

Meanwhile, could you add the variant to the method name as well in this PR?

[ofpiyush]

Version 7 of twirp fixed this issue.

Thank you for bringing it up @khevse