Slander of PyTrilinos feedstock is too broad

[guyer]

USAGE.rst says

trilinos/Trilinos#3203, but PyTrilinos on conda-forge presently only provides 12.10, which is limited to Python 2.x. :ref:PETSC is available for both :term:Python 3 and :term:Python 2.7.

In fact, PyTrilinos on conda-forge will install in Python 3.7 (and even 3.8 sometimes). That's still hopelessly behind the times, but at least it's Py3k.