mbedtls_test.py: Tell mypy to ignore mbed_host_tests

Since no typing stubs are available for mbed_host_tests.py, mypy
errors out on mbedtls_test.py with

    error: Skipping analyzing 'mbed_host_tests': found module but no type hints or library stubs

Ignore this import to get at least some benefit from mypy without
spending significant effort to write stubs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/scripts/mbedtls_test.py b/tests/scripts/mbedtls_test.py
index a5d0940..64f12bb 100755
--- a/tests/scripts/mbedtls_test.py
+++ b/tests/scripts/mbedtls_test.py
@@ -38,7 +38,7 @@
 import os
 import binascii
 
-from mbed_host_tests import BaseHostTest, event_callback # pylint: disable=import-error
+from mbed_host_tests import BaseHostTest, event_callback # type: ignore # pylint: disable=import-error
 
 
 class TestDataParserError(Exception):