Switch to libc.math import for infinity (#931)

* Switch to recommended import

* Consilidate imports

Author: stanmart

src/glum/_cd_fast.pyx

@@ -7,10 +7,9 @@
 # License: BSD 3 clause
 #         Substantial modifications by Ben Thompson <[email protected]>
 #
-from libc.math cimport fabs
+from libc.math cimport INFINITY, fabs
 cimport numpy as np
 import numpy as np
-from numpy.math cimport INFINITY
 
 from cython cimport floating
 from cython.parallel import prange