[python3] Mark remaining modules as py3-compatible

Mostly by using `api.file.read_proto()` to deserialize jsonpb files,
because it sorts keys, whereas using `api.file.read_text()` and then
`json_format.Parse()` doesn't sort keys, resulting in different key
orders between Python 2 and 3.

Also use `api.raw_io.output_text()` instead of `api.raw_io.output()`,
because `output()` returns a bytes object in Python 3, whereas
`output_text` returns a string in both Python 2 and Python 3.

Change-Id: Ia0e9ed076edfb3340c091db1ed75d9c2b9d57e4c
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/18520
Commit-Queue: Oliver Newman <olivernewman@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>