mention gmpy2

Author: skirpichev

peps/pep-0791.rst

@@ -183,6 +183,9 @@ Some possible additions, among those proposed in the initial discussion thread
   `python/cpython#120950 <https://github.com/python/cpython/issues/120950>`_.
 * ``fibonacci()`` --- `Fibonacci sequence <https://en.wikipedia.org/wiki/Fibonacci_sequence>`_.
 
+When proposed function already exists in the :pypi:`gmpy2`, we should prefer a
+compatible interface for the stdlib.
+
 
 Rejected ideas
 ==============