tests: update constraints for python 3.9 testing

[parthea]

Since Python 3.7 is no longer being tested following #1391 (see follow up in #1392), we should update the constraints files for python 3.9 testing to ensure that the minimum versions of dependencies stated in setup.py are compatible.

python-spanner/testing/constraints-3.7.txt

Lines 1 to 6 in f81fbd5

	# This constraints file is used to check that lower bounds
	# are correct in setup.py
	# List all library dependencies and extras in this file.
	# Pin the version to the lower bound.
	# e.g., if setup.py has "google-cloud-foo >= 1.14.0, < 2.0.0dev",
	# Then this file should have google-cloud-foo==1.14.0