tests: revert testing against prerelease deps on Python 3.9#406

tseaver merged 1 commit intomastergoogleapis/python-bigtable:masterfrom

revert-402-401-test-against-prerelease-deps-under-3.9googleapis/python-bigtable:revert-402-401-test-against-prerelease-deps-under-3.9Copy head branch name to clipboard