dtls.pyx: Declare language_level = 2

DTLSSocket/dtls.pyx

+# cython: language_level=2
 cimport tdtls
 from tdtls cimport dtls_context_t, dtls_handler_t, session_t, dtls_alert_level_t, dtls_credentials_type_t
 from libc.stdint cimport uint8_t