MAINT: Fix python version check in setup.py to require Python 3.8

Previously, this was not strictly enforced for the only purpose of
running LGTM (hoping it would be fixed before the 1.22 release).

Author: seberg

setup.py

@@ -30,8 +30,7 @@
 
 # Python supported version checks. Keep right after stdlib imports to ensure we
 # get a sensible error for older Python versions
-# This needs to be changed to 3.8 for 1.22 release, but 3.7 is needed for LGTM.
-if sys.version_info[:2] < (3, 7):
+if sys.version_info[:2] < (3, 8):
     raise RuntimeError("Python version >= 3.8 required.")