build,win: accept Python 3 if 2 is not available

If there is no Python 2 available, use Python 3. This allows to test
running configure with Python 3.

PR-URL: https://github.com/nodejs/node/pull/29236


Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>