Validate the APNS certificate at startup.

def _validate_apns_certificate(self, certfile):
		"""Validate the APNS certificate at startup."""

		try:
			with open(certfile, "r") as f:
				content = f.read()
				check_apns_certificate(content)
		except Exception as e:
			raise ImproperlyConfigured(
				"The APNS certificate file at %r is not readable: %s" % (certfile, e)
			)