Slightly refine protobuf and srptools base requirements (#902)
Slightly refine protobuf and srptools base requirements (#902)

* Restrict protobuf to below version 4

They decided not to relase version 4, and they've had releases since
then that quiet some warnings that are cluttering up logs.

* Make srptools a normal strict equality requirement

triple-equals disables all version string parsing, ands compares
versions like strings. The history (and documentation) say that other
versions should be supported.
postlund/pyatv,postlund/pyatv