[docs] [issue34538] Remove encouragement to author a base class for all Exception subclasses in a module

Change by Irit Katriel iritkatriel@yahoo.com:

---------- keywords: +easy versions: +Python 3.10, Python 3.9 -Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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