Make all protocols' param names pos-only in codecs.pyi

[sobolevn]

Closes #10657

[srittau]

Hmm, I'm reconsidering this in light of the primer hits and the fact that the docs state things like this:

class codecs.IncrementalEncoder(errors='strict')
Constructor for an IncrementalEncoder instance.

All incremental encoders must provide this constructor interface. They are free to add additional keyword arguments, but only the ones defined here are used by the Python codec registry.

encode
decode
The stateless encoding and decoding functions. These must be functions or methods which have the same interface as the encode() and decode() methods of Codec instances (see Codec Interface). The functions or methods are expected to work in a stateless mode.
incrementalencoder
incrementaldecoder
Incremental encoder and decoder classes or factory functions. These have to provide the interface defined by the base classes IncrementalEncoder and IncrementalDecoder, respectively. Incremental codecs can maintain state.

[srittau]

At least for the incremental coders. Since normal codecs are seemingly implemented with positional-only args in CPython, using pos-only protocols seems correct and able to catch errors.

[srittau]

Suggestion: Change the normal codecs, but leave the incremental coders for now and wait what for the discussion in python/cpython#109444.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉