Simplify float NaN check

Author: vnlitvinov

pandas/_libs/tslibs/nattype.pyx

@@ -1,7 +1,6 @@
 # -*- coding: utf-8 -*-
 
 from cpython cimport (
-    PyFloat_Check, PyFloat_AS_DOUBLE, PyComplex_Check,
     PyObject_RichCompare,
     Py_GT, Py_GE, Py_EQ, Py_NE, Py_LT, Py_LE)
 
@@ -12,7 +11,7 @@ PyDateTime_IMPORT
 
 import numpy as np
 cimport numpy as cnp
-from numpy cimport int64_t
+from numpy cimport int64_t, float64_t
 cnp.import_array()
 
 from pandas._libs.tslibs.np_datetime cimport (
@@ -715,12 +714,12 @@ NaT = c_NaT        # Python-visible
 cdef inline bint checknull_with_nat(object val):
     """ utility to check if a value is a nat or not """
     cdef:
-        double dval
+        float64_t fval
     if val is None or val is c_NaT:
         return True
-    if PyFloat_Check(val):
-        dval = val
-        return dval != dval
+    if isinstance(val, float):
+        fval = val
+        return fval != fval
     return util.is_nan(val)