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The random.Random class has a strange inheritance which is something uncommon in Python: it inherits from _random.Random which a concrete Mersenne Twister PRNG.
When a class inherit it, like random.SystemRandom, it should carefully override the right method.
I would expect to have something like a base class with abstract methods which raise NotImplementedError, and a concrete implementation which inherits from the base class and the Mersenne Twister C implementation.
A concrete issue is the how subclass handle the newly added randbytes() method:
https://bugs.python.org/issue40286#msg366860
The base class should implement randbytes() using getrandbits(), and the Mersenne Twister would override randbytes() with its fast implementation.
It would avoid the need for such method:
 def __init_subclass__(cls, /, **kwargs):
 """Control how subclasses generate random integers.
 The algorithm a subclass can use depends on the random() and/or
 getrandbits() implementation available to it and determines
 whether it can generate random integers from arbitrarily large
 ranges.
 """
 for c in cls.__mro__:
 if '_randbelow' in c.__dict__:
 # just inherit it
 break
 if 'getrandbits' in c.__dict__:
 cls._randbelow = cls._randbelow_with_getrandbits
 break
 if 'random' in c.__dict__:
 cls._randbelow = cls._randbelow_without_getrandbits
 break
It would also be nice if the base class would support a RNG which only produce bytes, like SystemRandom with os.urandom() and implement getrandbits() from that.
I also had concerns with the implementation of bpo-40282 "Allow random.getrandbits(0)":
https://github.com/python/cpython/pull/19539#pullrequestreview-393728231
It's maybe time to fix the class hierarchy.