Fall back to loading the ObjC runtime by name

ctypes.util.find_library() resolves a system library through the dyld shared
cache, which requires _dyld_shared_cache_contains_path(). Modules/_ctypes/
callproc.c guards that symbol with __builtin_available(iOS 14.0), so on iOS 13 -
the oldest release CPython's iOS support targets - find_library() returns None
for any library that exists only in the cache, and importing _ios_support fails
outright rather than degrading.

dyld resolves a bare library name against the cache by itself, so use that when
find_library() comes up empty. A genuine load failure is still reported as
ImportError.

Author: clementperon

Lib/_ios_support.py

@@ -10,12 +10,15 @@
 else:
     # ctypes is available. Load the ObjC library, and wrap the objc_getClass,
     # sel_registerName methods
-    lib = util.find_library("objc")
-    if lib is None:
-        # Failed to load the objc library
-        raise ImportError("ObjC runtime library couldn't be loaded")
+    # find_library() resolves a system library through the dyld shared cache,
+    # which needs _dyld_shared_cache_contains_path(); that is unavailable before
+    # iOS 14, so fall back to the bare name, which dyld resolves by itself.
+    lib = util.find_library("objc") or "libobjc.dylib"
 
-    objc = cdll.LoadLibrary(lib)
+    try:
+        objc = cdll.LoadLibrary(lib)
+    except OSError:
+        raise ImportError("ObjC runtime library couldn't be loaded")
     objc.objc_getClass.restype = c_void_p
     objc.objc_getClass.argtypes = [c_char_p]
     objc.sel_registerName.restype = c_void_p