Change the declaration of PyFloat_FromString to the Py3 signature and provide a fallback for Py2.

Closes https://github.com/cython/cython/issues/3909

Cython/Includes/cpython/float.pxd

 cdef extern from "Python.h":
+    """
+    #if PY_MAJOR_VERSION >= 3
+      #define __Pyx_PyFloat_FromString(obj)  PyFloat_FromString(obj)
+    #else
+      #define __Pyx_PyFloat_FromString(obj)  PyFloat_FromString(obj, NULL)
+    #endif
+    """
 
     ############################################################################
     # 7.2.3
@@ -21,7 +28,7 @@ cdef extern from "Python.h":
     # Return true if its argument is a PyFloatObject, but not a
     # subtype of PyFloatObject.
 
-    object PyFloat_FromString(object str, char **pend)
+    object PyFloat_FromString "__Pyx_PyFloat_FromString" (object str)
     # Return value: New reference.
     # Create a PyFloatObject object based on the string value in str,
     # or NULL on failure. The pend argument is ignored. It remains