Add XFAIL on test_negative_binomial under float32

This is a regression, likely caused by an updated NumPy/SciPy dependency.
Also see #5450

Author: michaelosthege

pymc3/tests/test_distributions.py

@@ -1082,6 +1082,10 @@ def modified_scipy_hypergeom_logcdf(value, N, k, n):
             {"N": NatSmall, "k": NatSmall, "n": NatSmall},
         )
 
+    @pytest.mark.xfail(
+        condition=(theano.config.floatX == "float32"),
+        reason="Fails on float32 due to a float casting problem in the logcdf",
+    )
     def test_negative_binomial(self):
         def scipy_mu_alpha_logpmf(value, mu, alpha):
             return sp.nbinom.logpmf(value, alpha, 1 - mu / (mu + alpha))