"py-" prefix shouldn't be needed for Python-based ports that are programs, not libraries

[Yuri]

I think that this section 
https://wiki.freebsd.org/Python/PortsPolicy#Prefixing_.28py-.2A.29 
should be amended:

Examples of compelling cases:
* The port is a program, and not a library.


There is no need to have flavors for Python ports that build only 
programs and don't build libraries.


Thanks,

Yuri