mail/py-python-slimta: Unbreak import with FreeBSD Python

In order to retain compatibility with Python 2, mail/py-python-slimta mandates
an older version of mail/py-pysasl than what is in Ports. However, lang/python27
has expired so there should be no compatibility issues with FreeBSD.

Reviewed by: 0mp (mentor)
Approved by: 0mp (mentor)
Differential Revision: https://reviews.freebsd.org/D28051