closes bpo-39605: Fix some casts to not cast away const. (GH-18453)

gcc -Wcast-qual turns up a number of instances of casting away constness of pointers. Some of these can be safely modified, by either:

Adding the const to the type cast, as in:

-    return _PyUnicode_FromUCS1((unsigned char*)s, size);
+    return _PyUnicode_FromUCS1((const unsigned char*)s, size);

or, Removing the cast entirely, because it's not necessary (but probably was at one time), as in:

-    PyDTrace_FUNCTION_ENTRY((char *)filename, (char *)funcname, lineno);
+    PyDTrace_FUNCTION_ENTRY(filename, funcname, lineno);

These changes will not change code, but they will make it much easier to check for errors in consts

Python/sysmodule.c

@@ -204,7 +204,7 @@ PySys_Audit(const char *event, const char *argFormat, ...)
 
     /* Dtrace USDT point */
     if (dtrace) {
-        PyDTrace_AUDIT((char *)event, (void *)eventArgs);
+        PyDTrace_AUDIT(event, (void *)eventArgs);
     }
 
     /* Call interpreter hooks */