I noticed that StreamReaderProtocol is defined like this:

class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
    ...

but FlowControlMixin already inherits protocols.Protocol:

class FlowControlMixin(protocols.Protocol):
    ...

It seems redundant that StreamReaderProtocol has protocols.Protocol as a second base class (is this an oversight or I'm missing some hidden reasoning behind it in which case it might be useful to have it mentioned in a comment to avoid confusion).

You are right, but the existing inheritance is also correct (while a little redundant).

Sorry, I don't think we should do anything: don't fix if not broken.

While it may not be affected at a functional level (at least not in a way I can see right now), it is confusing when reading that code.

Isn't clarity and avoiding confusion a desired trait for core python code?

What about all these recommendations from the Zen of Python?

Beautiful is better than ugly.
Simple is better than complex.
Flat is better than nested.
Readability counts.
If the implementation is hard to explain, it's a bad idea.

Fixing a non-broken thing is not a world-class idea, I think.