mqtt test: remove unused import

.github/workflows/python.yml

@@ -42,6 +42,7 @@ jobs:
         - 3.6
         - 3.7
         - 3.8
+      fail-fast: false
     steps:
     - uses: actions/checkout@v1
     - uses: actions/setup-python@v1

tests/test_mqtt.py

@@ -16,7 +16,6 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 import logging
-import socket
 import unittest.mock
 
 import pytest