Remove mention of isometric from __all__

[Ibrahim2750mi]

>>> 'isometric' in arcade.__all__
True
>>> arcade.isometric
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'arcade' has no attribute 'isometric'

Relevant conversation: https://discord.com/channels/458662222697070613/458662458198982676/1087269080072978523