Fixes the exception below that happened for function-markers:
```
Traceback (most recent call last):
File "/usr/bin/../lib/kitty/kitty/boss.py", line 540, in dispatch_special_key
return self.dispatch_action(key_action)
File "/usr/bin/../lib/kitty/kitty/boss.py", line 612, in dispatch_action
passthrough = f(*key_action.args)
File "/usr/bin/../lib/kitty/kitty/window.py", line 615, in toggle_marker
self.screen.set_marker(marker_from_spec(ftype, spec, flags))
File "/usr/bin/../lib/kitty/kitty/marks.py", line 92, in marker_from_spec
return marker_from_function(runpy.run_path(path, run_name='__marker__').marker)
```