tests: revert testing against prerelease deps on Python 3.9#426

gcf-merge-on-green[bot] merged 2 commits intomastergoogleapis/python-firestore:masterfrom

revert-415-414-test-against-prerelease-deps-under-3.9googleapis/python-firestore:revert-415-414-test-against-prerelease-deps-under-3.9Copy head branch name to clipboard