[petsc-dev] self.download = ['https://bitbucket.org/mpi4py/mpi4py/downloads/mpi4py-3.1.3.tar.gz', is archived

[Barry Smith]

The bitbucket readme says that this has been archived and active development is 
at Github. I don't think PETSc's configure should be referencing the outdated 
location at bitbucket.