Make various internal _testbuffer symbols static. (GH-8160)

(cherry picked from commit 6cfe45a)

Co-authored-by: Benjamin Peterson <benjamin@python.org>

Author: miss-islington

Modules/_testbuffer.c

@@ -8,13 +8,13 @@
 
 
 /* struct module */
-PyObject *structmodule = NULL;
-PyObject *Struct = NULL;
-PyObject *calcsize = NULL;
+static PyObject *structmodule = NULL;
+static PyObject *Struct = NULL;
+static PyObject *calcsize = NULL;
 
 /* cache simple format string */
 static const char *simple_fmt = "B";
-PyObject *simple_format = NULL;
+static PyObject *simple_format = NULL;
 #define SIMPLE_FORMAT(fmt) (fmt == NULL || strcmp(fmt, "B") == 0)
 #define FIX_FORMAT(fmt) (fmt == NULL ? "B" : fmt)