lint: Run mypy with --show-error-codes

When using mypy ignore directives, the error code needs to be specified.
Somehow mypy doesn't print it by default...

test/lint/lint-python.sh

@@ -102,7 +102,7 @@ if ! PYTHONWARNINGS="ignore" flake8 --ignore=B,C,E,F,I,N,W --select=$(IFS=","; e
     EXIT_CODE=1
 fi
 
-if ! mypy --ignore-missing-imports $(git ls-files "test/functional/*.py" "contrib/devtools/*.py"); then
+if ! mypy --ignore-missing-imports --show-error-codes $(git ls-files "test/functional/*.py" "contrib/devtools/*.py"); then
     EXIT_CODE=1
 fi