Incorrect usage of slots and class attribute

[bergkvist]

Problem:

remerkleable/remerkleable/virtual.py

Lines 23 to 26 in 91ed092

	_is_leaf: Optional[bool] = None
	_left: Optional[Node] = None
	_right: Optional[Node] = None
	__slots__ = '_root', '_src', '_is_leaf', '_left', '_right'

Error:

ValueError: '_is_leaf' in __slots__ conflicts with class variable

If you set the default values to None in __init__ instead, it will fix the problem.

Python 3.6, 3.7, 3.8, 3.9, 3.10, 3.11 all complain about __slots__ conflicts when you give attributes a default value.

See: python/cpython#106067