[Python] How do I add a type hint for a WriteableBuffer parameter?

I'm trying to add a parameter type to the readinto() method declared in a custom class that derives from RawIOBase, like this:

from io import RawIOBase

class Reader(RawIOBase):
def readinto(self, buf: bytearray) -> int:
pass # actual implementation omitted


But pyright complains:

io.py:6:9 - error: Method "readinto" overrides class "_RawIOBase" in an incompatible manner
Parameter 2 type mismatch: base parameter is type "WriteableBuffer", override parameter is type "bytearray"
"Buffer" is not assignable to "bytearray" (reportIncompatibleMethodOverride)
1 error, 0 warnings, 0 informations



How do I fix this? Note: I know I can remove the type hint entirely. I want to assign it the correct type instead.

I'm using Python 3.13.3 and pyright 1.1.400.

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