stdout_callback is used to flow progress information from the conversion
to some front-end. It was always used in tandem with printing to the
terminal (which is kind of a front-end). So it made sense to put them
always together.
Due to a bump in our Python dependencies, we now install Mypy 1.1.1
instead of 0.982. This change triggered the following errors:
* Incompatible default for argument <a> (default has type
None, argument has type <t>):
Mypy further explains here that PEP 484 prohibits implicit Optional,
so we need to make these types explicit Optional.
* Unused "type: ignore" comment, use narrower [method-assign] instead of
[assignment]:
Mypy has specialized some of its lints, meaning that we should switch
to the newer variants.
Also, it detected several other small inconsistencies. We fix all of
these errors in this commit.