typing.io is not referenced in either the tests or the implementation for pydoc. What happens is basically: typing.io is a class, so its derived from object. When formatting the docstring for object, the formatter queries the __module__ attribute of all sub-classes of object, including typing.io, so that it can include the sub-classes in its output:

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Help on class object in module builtins:

class object
 |  The base class of the class hierarchy.
 |  
 |  When called, it accepts no arguments and returns a new featureless
 |  instance that has no instance attributes and cannot be given any.
 |  
 |  Built-in subclasses:
 |      anext_awaitable
 |      async_generator
 |      async_generator_asend
 |      async_generator_athrow
 |      ... and 88 other subclasses
 |  
[...]

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Before the PR this failed, as accessing any typing.io.__module__ printed the warning.