[jwcrypto] Work around stubtest issues

See python/mypy#20160 for details.

Closes: #14893

Author: srittau

stubs/jwcrypto/@tests/stubtest_allowlist.txt

@@ -4,3 +4,6 @@ jwcrypto.tests-cookbook
 
 # even if the deprecated decorator is applied, the attribute is not present
 jwcrypto.jwt.JWTMissingKeyID.__deprecated__
+
+# https://github.com/python/mypy/issues/20160
+jwcrypto.jwt.JWTMissingKeyID.__init_subclass__