python: key expiration datetime stamp tests

* Changed the expiration date for the generated test key to NYE this
  century, rather than the NYE this millennium as originally suggested
  in job #3815.
* This covers the lifetimes of current users (except, maybe, some very
  healthy millennials) as well as the 32-bit clock end date in 2038;
  without falling foul of OpenPGP's 2106 expiration.

lang/python/tests/t-callbacks.py

@@ -88,7 +88,7 @@ Name-Real: Joe Tester
 Name-Comment: with stupid passphrase
 Name-Email: joe+gpg@example.org
 Passphrase: Crypt0R0cks
-Expire-Date: 2020-12-31
+Expire-Date: 2099-12-31
 </GnupgKeyParms>
 """