gh-114099: Add test exclusions to support running the test suite on iOS (#114889)

Add test annotations required to run the test suite on iOS (PEP 730).

The majority of the change involve annotating tests that use subprocess,
but are skipped on Emscripten/WASI for other reasons, and including
iOS/tvOS/watchOS under the same umbrella as macOS/darwin checks.

`is_apple` and `is_apple_mobile` test helpers have been added to
identify *any* Apple platform, and "any Apple platform except macOS",
respectively.

Lib/test/test_urllib2.py

@@ -1,6 +1,7 @@
 import unittest
 from test import support
 from test.support import os_helper
+from test.support import requires_subprocess
 from test.support import warnings_helper
 from test import test_urllib
 from unittest import mock
@@ -998,6 +999,7 @@ class HandlerTests(unittest.TestCase):
 
         file_obj.close()
 
+    @requires_subprocess()
     def test_http_body_pipe(self):
         # A file reading from a pipe.
         # A pipe cannot be seek'ed.  There is no way to determine the