Ignore venv from stylelint

`coverage` ships with a `style.scss`, which `stylelint` attempts to lint.

.stylelintrc.yaml

@@ -1,5 +1,6 @@
 ignoreFiles:
   - node_modules
   - build/**/*
+  - venv/**/*
 extends:
   - '@wagtail/stylelint-config-wagtail'