BLD: address build warnings (#30639)

Author: jbrockmendel

pandas/_libs/algos_take_helper.pxi.in

@@ -116,7 +116,7 @@ def take_2d_axis0_{{name}}_{{dest}}(ndarray[{{c_type_in}}, ndim=2] values,
 
     IF {{True if c_type_in == c_type_out != "object" else False}}:
         cdef:
-            {{c_type_out}} *v
+            const {{c_type_out}} *v
             {{c_type_out}} *o
 
         # GH#3130

pandas/_libs/hashtable.pyx

@@ -1,7 +1,7 @@
 cimport cython
 
 from cpython.ref cimport PyObject, Py_INCREF
-from cpython.mem cimport PyMem_Malloc, PyMem_Realloc, PyMem_Free
+from cpython.mem cimport PyMem_Malloc, PyMem_Free
 
 from libc.stdlib cimport malloc, free
 

pandas/_libs/tslibs/conversion.pxd

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-from cpython.datetime cimport datetime, tzinfo
+from cpython.datetime cimport datetime
 
 from numpy cimport int64_t, int32_t