bpo-40756: Default second argument of LoggerAdapter.__init__ to None

[arturoescaip]

The 'extra' argument is not always used by custom logger adapters. For
example:

class IndentAdapter(logging.LoggerAdapter):
    def process(self, msg, kwargs):
        indent = kwargs.pop(indent, 1)
        return ' ' * indent + msg, kwargs

It is cleaner and friendlier to default the 'extra' argument to None
instead of either forcing the subclasses of LoggerAdapter to pass a None
value directly or to override the constructor.

This change is backward compatible because existing calls to
LoggerAdapter.__init__ are already passing a value for the second
argument.

https://bugs.python.org/issue40756

Automerge-Triggered-By: @vsajip

[arturoescaip]

@vsajip This is the new version of #20114 after botching that pull request 😞

It looks like the blurb tool required a bpo number, so I created one.