fix WhiteNoise subclassing from Covariance (pymc-devs#6673)

Since pymc-devs#6458, Covariance is now the base class for kernels/covariance
functions with input_dim and active_dims, which does not include
WhiteNoise and Constant kernels.

Author: dehorsley

pymc/gp/cov.py

@@ -386,7 +386,7 @@ def full(self, X, Xs=None):
             return pt.alloc(self.c, X.shape[0], Xs.shape[0])
 
 
-class WhiteNoise(Covariance):
+class WhiteNoise(BaseCovariance):
     r"""
     White noise covariance function.