Fix crash on Catalina.
https://github.com/ronaldoussoren/pyobjc/issues/265
https://github.com/ronaldoussoren/pyobjc/commit/8c97708502f486e1e5fc43a5252e61f9ab282650
https://github.com/ronaldoussoren/pyobjc/commit/76b4bb2d76d33961821c6cc0d88c7031c62805c2
--- Modules/objc/method-signature.m.orig	2019-10-16 04:40:21.000000000 -0500
+++ Modules/objc/method-signature.m	2020-08-29 03:48:31.000000000 -0500
@@ -628,10 +628,16 @@
 #endif
         if (d && PyObject_IsTrue(d)) {
             if (descr == NULL || (descr->tmpl && !descr->alreadyRetained)) return -2;
-            descr->alreadyRetained = YES;
+            // descr may be loaded into read-only memory, so only
+            // write if truly necessary
+            if (!descr->alreadyRetained)
+                descr->alreadyRetained = YES;
         } else {
             if (descr == NULL || (descr->tmpl && descr->alreadyRetained)) return -2;
-            descr->alreadyRetained = NO;
+            // descr may be loaded into read-only memory, so only
+            // write if truly necessary
+            if (descr->alreadyRetained)
+                descr->alreadyRetained = NO;
         }
     }