docs: def find_files(self):
          '''Find files in `paths` which match valid extensions'''
          for path in self.paths:
              for subpath, dirs, files in os.walk(path):
                  for filename in files:
                      (name, ext) = os.path.splitext(filename)
                      if ext in self.extensions:
                          yield os.path.join(subpath, filename)