py-sympy have depenence of /math/py-mpmath. It should be included in the Makefile
A commit references this bug: Author: wen Date: Thu Apr 28 02:09:10 UTC 2016 New revision: 414153 URL: https://svnweb.freebsd.org/changeset/ports/414153 Log: - Add missing RUN_DEPENDS PR: 209076 Submitted by: jjachuf@gmail.com Changes: head/math/py-sympy/Makefile