The call-iterator concept is also an iterator

[macro1]

Description of change

Fixes #50 by adding __next__() as a method on CallIterator.

Internally, it seems grpc._channel._MultiThreadedRendezvous is used, which also has some other capabilities. However, as they aren't reflected in the documentation and this interface can be defined differently depending on usage, this does seem to be the right change.

Minimum Reproducible Example

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main.py

# Full python code to reproduce
if __name__ == "__main__":
    ...

run.sh

#!/usr/bin/env bash
set -o errexit -o nounset -o pipefail
python -m venv venv
source ./venv/bin/activate
pip install $INSERT_DEPENDENCIES_HERE
python main.py

Checklist:

• I have verified my MRE is sufficient to demonstrate the issue and solution by attempting to execute it myself
  • OR I believe my contribution is minor enough that a typing test is sufficient.
• I have added tests to typesafety/test_grpc.yml for all APIs added or changed by this PR
• I have removed any code generation notices from anything seeded using mypy-protobuf.