_codecs: fix various issues (#3649)
_codecs: fix various issues (#3649)

* _codecs: mark positional-only args as such
* _codecs: rename __data to __str where relevant
* _codecs: fix utf_16_ex_decode and utf_32_ex_decode
* _codecs: fix default value of charmap_encode
* _codecs: fix availability of charbuffer_encode

euresti/typeshed