Add support for guild banners

Document banner attribute of Guild and Invite

Update discord/utils.py

Co-Authored-By: SnowyLuma <38926001+SnowyLuma@users.noreply.github.com>

discord/utils.py

@@ -294,8 +294,8 @@ async def sane_wait_for(futures, *, timeout, loop):
         raise asyncio.TimeoutError()
 
 def valid_icon_size(size):
-    """Icons must be power of 2 within [16, 2048]."""
-    return not size & (size - 1) and size in range(16, 2049)
+    """Icons must be power of 2 within [16, 4096]."""
+    return not size & (size - 1) and size in range(16, 4097)
 
 class SnowflakeList(array.array):
     """Internal data storage class to efficiently store a list of snowflakes.