Update distribution.py

poisson_exact() function for removal cogent3#1734

src/cogent3/maths/stats/distribution.py

@@ -24,6 +24,7 @@
     ndtri,
 )
 
+from cogent3.util import warning as c3warn
 
 # ndtri import b/c it should be available via this module
 
@@ -59,11 +60,9 @@ def poisson_low(successes, mean):
     return pdtr(successes, mean)
 
 
-def poisson_exact(successes, mean):
-    """Returns Poisson probablity for exactly Pr(X=successes).
-
-    Formula is e^-(mean) * mean^(successes) / (successes)!
-    """
+@c3warn.deprecated_callable(version="2024.9", reason="use scipy.stats.poisson.pmf()instead", is_discontinued=True)
+def poisson_exact(successes, mean): # pragma: no cover
+    """being removed""" 
     if successes == 0:
         return pdtr(0, mean)
     elif successes < mean:  # use left tail