Switch the default Python version to 3.8

3.7 is currently in security fixes only mode, but 3.8 will
continue to recieve a few more months of normal bug fixes.
This should give us time to prep the tree for 3.9 (a few packages
are not happy with 3.9 yet).

3.8 might be more problematic than 3.7 for Linuxes using older compilers
where shm_open isn't detected probably (however this problem doesn't
seem to be entirely absent in 3.7, as shown by certain packages like
mozjs*?). According to the CentOS 7 folks on tech-pkg this should be
possible to work around by using a newer compiler.

(nia)