[python] remove workaround for UnboundLocalError
in early stopping callback for Python 2
#4855
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Python 2 support was dropped for this project in #3581.
With Python 3 we can use
nonlocal
statement instead of wrapping variables into lists as a workaround forUnboundLocalError
in nested function scopes.This workaround was introduced in #1895 (comment) when we had to support Python 2.
Found this during investigating the failures in #4846.