biology/libsbml: limit supported python versions
File "/wrkdirs/usr/ports/biology/libsbml/work/libsbml-5.19.0/src/bindings/python/doc-converter/rewrite_pydoc.py", line 121, in <module> from formatter import NullWriter, AbstractFormatter
ModuleNotFoundError: No module named 'formatter'
Approved by: portmgr blanket