Silence pylint

Silence pylint in specific places where we're doing slightly unusual
or dodgy, but correct.
diff --git a/tests/scripts/mbedtls_test.py b/tests/scripts/mbedtls_test.py
index c702765..ac2912d 100755
--- a/tests/scripts/mbedtls_test.py
+++ b/tests/scripts/mbedtls_test.py
@@ -37,7 +37,8 @@
 import re
 import os
 import binascii
-from mbed_host_tests import BaseHostTest, event_callback
+
+from mbed_host_tests import BaseHostTest, event_callback # pylint: disable=import-error
 
 
 class TestDataParserError(Exception):