add cpplint script to flake8 excludes

Author: ivikhrev

.flake8

@@ -5,6 +5,8 @@ exclude=
     # generated files
     demos/speech_recognition_deepspeech_demo/python/ctcdecode-numpy/ctcdecode_numpy/impl.py,
     models/public/mozilla-deepspeech-0.8.2/mds_convert_utils/memmapped_file_system_pb2.py,
+    # 3rd party files
+    cmake/cpplint/cpplint.py
 
 select=
     # flake8-comprehensions issues