m4/python: Scan for python 3.8 as well

* m4/python.m4: Scan for python 3.8 as well.

It's not clear to me why python3.8 should be commented out of the
python path search. This change simplifies and normalizes the search
for modern versions of python 3.

• Signed-Off-By: Daniel Kahn Gillmor <dkg@fifthhorseman.net>