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/marshal.c

@@ -734,7 +734,7 @@ r_byte(RFILE *p)
     else {
         const char *ptr = r_string(1, p);
         if (ptr != NULL)
-            c = *(unsigned char *) ptr;
+            c = *(const unsigned char *) ptr;
     }
     return c;
 }