does the windows package include its own self sufficient python runtime, or does it rely on a specific 3rd party python being installed ? if the latter, is python 2.x available or just 3.x ?
As I understand plpython on windows is only adapter to ActiveState Python 2 or 3. EDB installer doesn't include in distribution package Python runtime.
Windows doesn't include preinstalled python of any kind. While installing plpython you must to download and preinstall ActiveState Python version 3 (because EDB installer doesn't contain Python 2 adapter dll).
Windows installer has another problem. It requires specific version of ActiveState Python, but ActiveState allows to download those versions of Python only limited time. After it releases new version old version becomes available only to paying ActiveState customers effectively prohibiting plpython to Postgres Windows customers who discover their need of plpython too late.