Add py3-* packages for toposort and pyPEG2.

The port for ARPC (https://github.com/NuxiNL/arpc) I am preparing comes
with a script called 'aprotoc' that depends on these Python modules.
Ports that default to Python 2 have already been added, but now we
should add Python 3 flavours as well.

Reviewed by: koobs
Approved by: portmgr (bdrewery)
Differential Revision: https://reviews.freebsd.org/D12068