Update xtrim type annotation (#2093)

Author: richli

redis/commands/core.py

@@ -3819,7 +3819,7 @@ def xrevrange(
  def xtrim(
  self,
  name: KeyT,
- maxlen: int,
+ maxlen: Union[int, None],
  approximate: bool = True,
  minid: Union[StreamIdT, None] = None,
  limit: Union[int, None] = None,