Add Python 3.7 as supported for tensorflow-serving-api package.
Add Python 3.7 as supported for tensorflow-serving-api package.

The API package itself is a collection of .proto files, and should be
compatible with Python 3.7. Underlying dependencies: protobuf and grpcio
both support Python 3.7, so we can too.

Fixes #1640

PiperOrigin-RevId: 313979135

tensorflow/serving,tensorflow/serving,tensorflow/serving,tensorflow/serving