net/py-mpi4py: add a slave port depending on MPICH

Sometimes we need mpi4py packaged against mpich, with Fortran enabled.

Do not create a flavor, because there already exist flavors for Python
versions, therefore a slave port seems more appropriate.

Once committed, I shall add a line in Mk/Uses/mpi.mk to grab the right
dependency according to the choice of MPI.

Bump PORTVERSION for people using net/py-mpi4py built from ports with
the non-default MPICH option.

Also change WWW: the project on bitbucket has been archived.

PR: 272949
Approved by: wen@ (maintainer)