old_contents: from header_filter.matchers import Header  # noqa: F401
  from header_filter.middleware import HeaderFilterMiddleware  # noqa: F401
  from header_filter.rules import Enforce, Forbid  # noqa: F401
  
new_contents: from header_filter.matchers import Header, HeaderRegexp  # noqa: F401
  from header_filter.middleware import HeaderFilterMiddleware  # noqa: F401
  from header_filter.rules import Enforce, Forbid  # noqa: F401
  
subject: Allow HeaderRegexp to be imported directly from header_filter package.