docs: Use better links for mpi4py

I asked Lisandro what link we should use for mpi4py; he asked that we
use https://mpi4py.github.io/.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

docs/news/news-v5.0.x.rst

@@ -75,7 +75,7 @@ Open MPI version 5.0.0rc12
     dynamic shared objects.
   - Fix ``MPI_Session_init()`` bug if all previous sessions are
     finalized.
-  - Fix `mpi4py <https://mpi4py.readthedocs.io/>`_ hang in
+  - Fix `mpi4py <https://mpi4py.github.io/>`_ hang in
     ``MPI_Intercomm_create_from_groups()``.
   - Fix finalization segfault with OSHMEM 4.1.5.
   - Improve AVX detection. Fixes ``op/avx`` link failure with the

docs/tuning-apps/dynamic-loading.rst

@@ -38,7 +38,7 @@ operating systems use other suffixes, such as ``.so``):
 
 .. note:: The above is just an example showing dynamic loading.  If
           you want to use MPI in Python, you are much better off using
-          `MPI4Py <https://mpi4py.readthedocs.io/en/stable/>`_.
+          `MPI4Py <https://mpi4py.github.io/>`_.
 
 Other scripting languages should have similar options when dynamically
 loading shared libraries.