bpo-43971: Add spaces around annotated arg default '=' (GH-25702)

Result: "quantity_on_hand: int = 0".

Author: moselhy

Doc/library/dataclasses.rst

@@ -33,7 +33,7 @@ using :pep:`526` type annotations. For example this code::
 
 Will add, among other things, a :meth:`__init__` that looks like::
 
- def __init__(self, name: str, unit_price: float, quantity_on_hand: int=0):
+ def __init__(self, name: str, unit_price: float, quantity_on_hand: int = 0):
  self.name = name
  self.unit_price = unit_price
  self.quantity_on_hand = quantity_on_hand