On a local branch  with some modified docs
```
Traceback (most recent call last):
  File "/Users/bussonniermatthias/dev/cpython/Doc/venv/lib/python3.8/site-packages/sphinx/cmd/build.py", line 284, in build_main
    app.build(args.force_all, filenames)
  File "/Users/bussonniermatthias/dev/cpython/Doc/venv/lib/python3.8/site-packages/sphinx/application.py", line 337, in build
    self.builder.build_update()
  File "/Users/bussonniermatthias/dev/cpython/Doc/venv/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 324, in build_update
    self.build(to_build,
  File "/Users/bussonniermatthias/dev/cpython/Doc/venv/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 392, in build
    self.finish()
  File "/Users/bussonniermatthias/dev/cpython/Doc/tools/extensions/suspicious.py", line 118, in finish
    self.warn('Found %s/%s unused rules:' %
AttributeError: 'CheckSuspiciousMarkupBuilder' object has no attribute 'warn'
```

My guess is that it should be self.logger.warn, and not self.warn.