[3.7] bpo-35489: Use "const Py_UNICODE *" for the Py_UNICODE converter in AC. (GH-11150). (GH-11151)

(cherry picked from commit afb3e71a1710c444fbe789b51df43ee16ee9ede7)
diff --git a/Tools/clinic/clinic.py b/Tools/clinic/clinic.py
index b704b5a..0ba8cab 100755
--- a/Tools/clinic/clinic.py
+++ b/Tools/clinic/clinic.py
@@ -2824,7 +2824,7 @@
 @add_legacy_c_converter('Z', accept={str, NoneType})
 @add_legacy_c_converter('Z#', accept={str, NoneType}, zeroes=True)
 class Py_UNICODE_converter(CConverter):
-    type = 'Py_UNICODE *'
+    type = 'const Py_UNICODE *'
     default_type = (str, Null, NoneType)
     format_unit = 'u'