Stop AttributeError from SpriteList._atlas and link Lazy SpriteList doc

[pushfoo]

Changes

Related to #1882

• Allow None for SpriteList._atlas and SpriteList.atlas
• Update unit tests
• Comment and clarify lazy-related unit tests
• Update SpriteList docstrings:
  • Link the programming guide page on lazy sprite lists
  • Remove or add detail as necessary
• Nasty performance / typing tricks to avoid overhead from typing.cast

How to test

Code

1. ./make.py test
2. Run your choice of sprite examples to look for any issues

Doc

1. ./make.py clean
2. ./make.py html (optional, workaround for occasional build loop issue)
3. ./make.py serve
4. Navigate to http://localhost:8000/api_docs/api/sprite_list.html#arcade.SpriteList
   1. Is the wording clear?
   2. Do the draw and initialize method links go to SpriteList's methods?
   3. Does the Lazy SpriteLists link go to relevant doc?
5. Navigate to http://localhost:8000/api_docs/api/sprite_list.html#arcade.SpriteList.draw
   1. Is the wording clear?
   2. Does the initialize method link go to SpriteList.initialize?
   3. Does the Lazy SpriteLists link go to relevant doc?
6. Navigate to http://localhost:8000/api_docs/api/sprite_list.html#arcade.SpriteList.initialize
   1. Is the wording clear?
   2. Does the draw method link go to SpriteList.draw?
   3. Does the Lazy SpriteLists link go to relevant doc?

[pushfoo]

@bunny-therapist How does this look? On my system and the unit test runner, it seems to work. Your example also runs fine now:

import arcade

class IssueDemonstrationWindow(arcade.Window):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        sl = arcade.SpriteList(lazy=True)
        sl.atlas


window = IssueDemonstrationWindow()
arcade.run()

My remaining concern is that we may want to expose SpriteList._initialized as a read-only initialized property, but that could be left out or put in another PR.