Pin Python version to 3.7 in integration tests
Pin Python version to 3.7 in integration tests

Python 3.9 is incompatible with the Google Cloud SDK at the moment. It supports 3.6-3.8, so we pin to 3.7 to be consistent with the version of Pytohn used by the workflows that build the Firebase SDK.

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk