add an example of how circular imports can be handled

The type name needs to be quoted so it is evaluated after the module
has been initialized.

In the future when we drop support for python 3.6, and mypy is updated,
we will be able to remove the quotes and use the following at
the top of the file instead:

from __future__ import annotations

anki/cards.py

@@ -5,6 +5,7 @@ import pprint
 import time
 from typing import Any, Dict, Optional
 
+import anki  # pylint: disable=unused-import
 from anki.consts import *
 from anki.hooks import runHook
 from anki.notes import Note
@@ -27,9 +28,8 @@ class Card:
     timerStarted: Optional[float]
     lastIvl: Optional[int]
 
-    def __init__(self, col, id: Optional[int] = None) -> None:
-        from anki.collection import _Collection
-        self.col: _Collection = col
+    def __init__(self, col: "anki.collection._Collection", id: Optional[int] = None) -> None:
+        self.col = col
         self.timerStarted = None
         self._qa = None
         self._note = None