Add the remove indirection back again

See Textualize#2854 -- turns out that it's
still an issue, but on Python 3.7 but not Python 3.11 (as mentioned in the
linked issue, I've not tested other versions yet).

src/textual/widgets/_toast.py

@@ -67,7 +67,13 @@ def __init__(
 
     def _on_mount(self, _: Mount) -> None:
         """Set the time running once the toast is mounted."""
-        self.set_timer(self._timeout, self.remove)
+        # https://github.com/Textualize/textual/issues/2854
+        self.set_timer(self._timeout, self._expire)
+
+    def _expire(self) -> None:
+        """Remove the toast once the timer has expired."""
+        # https://github.com/Textualize/textual/issues/2854
+        self.remove()
 
 
 class Rack(Container, inherit_css=False):