python: Fix a test for newer GnuPG (>= 2.3).

* lang/python/tests/t-quick-subkey-creation.py: Specify RSA.

Using GnuPG 2.3, a key with default (ed25519/cv25519) cannot have some
combination of capabilities (e.g., "encr auth").

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>