python: Fix typo in DecryptionError exception.

* lang/python/src/errors.py (DecryptionError): Rename from
DeryptionError.
(UnsupportedAlgorithm): Use DecryptionError.

• GnuPG-bug-id: T4478
• Reported-by: Jan Girlich
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>