editor: optionally the editor to use.  Defaults to automatic
                   detection.
    :param env: environment variables to forward to the editor.
    :param require_save: if this is true, then not saving in the editor
                         will make the return value become `None`.
    :param extension: the extension to tell the editor about.  This defaults
                      to `.txt` but changing this might change syntax
                      highlighting.
    :param filename: if provided it will edit this file instead of the
                     provided text contents.  It will not use a temporary
                     file as an indirection in that case.
    r