bpo-36641: Add "const" to PyDoc_VAR macro (GH-12854)

It reduces "data" segment in python about 200KB.

Include/pymacro.h

@@ -67,7 +67,7 @@
 
 
 /* Define macros for inline documentation. */
-#define PyDoc_VAR(name) static char name[]
+#define PyDoc_VAR(name) static const char name[]
 #define PyDoc_STRVAR(name,str) PyDoc_VAR(name) = PyDoc_STR(str)
 #ifdef WITH_DOC_STRINGS
 #define PyDoc_STR(str) str

Misc/NEWS.d/next/C API/2019-04-16-21-18-19.bpo-36641.pz-DIR.rst

+:c:macro:`PyDoc_VAR(name)` and :c:macro:`PyDoc_STRVAR(name,str)` now create
+``static const char name[]`` instead of ``static char name[]``.  Patch by Inada Naoki.

Modules/_ssl.c

@@ -557,7 +557,7 @@ SSLError_str(PyOSErrorObject *self)
 
 static PyType_Slot sslerror_type_slots[] = {
     {Py_tp_base, NULL},  /* Filled out in module init as it's not a constant */
-    {Py_tp_doc, SSLError_doc},
+    {Py_tp_doc, (void*)SSLError_doc},
     {Py_tp_str, SSLError_str},
     {0, 0},
 };