Reject too-old Python versions a bit sooner.

Reject too-old Python versions a bit sooner.

Commit 04aad4018 added this check after the search for a Python shared
library, which seems to me to be a pretty unfriendly ordering.  The
search might fail for what are basically version-related reasons, and
in such a case it'd be better to say "your Python is too old" than
"could not find shared library for Python".