Python ports are supposed to be prefixed with py-, and the binary packages have the Python version in the prefix as well, like py27-.
This is accomplished by setting PKGNAMEPREFIX in the following way, using security/py-onetime's Makefile as an example:
PORTNAME= onetime
PORTVERSION= 1.81
CATEGORIES= security
MASTER_SITES= http://www.red-bean.com/onetime/
PKGNAMEPREFIX= ${PYTHON_PKGNAMEPREFIX}
This will result in the following pkg:
root@dozer /# pkg info py27-onetime py27-onetime-1.81 Name : py27-onetime Version : 1.81 Installed on : Sat Sep 2 22:43:01 2017 CEST Origin : security/py-onetime
Note that the port's directory shall have the py-, the PORTNAME shall not be prefixed (this is taken care of by PKGNAMEPREFIX), and the resulting packages will be prefixed with pyXX- where XX is the Python version.