HPCC-8788 Require Python 2.7 to build pyembed

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4155 from richardkchapman/python-checkversion

HPCC-8788 Require Python 2.7 to build pyembed

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>