improve stubs for _codecs (#1219)
improve stubs for _codecs (#1219)

* improve stubs for _codecs

I noticed that many of these used AnyStr in a context where it doesn't make
sense. Found a few more things after reviewing _codecsmodule.c:
- Some Windows-only functions were omitted
- A few argument counts and names and return types were wrong

The Python 2 _codecs stub has many of the same issues, but the module is also
pretty similar between Python 2 and 3. I'm planning to send another PR after this
one to merge the two and move the stub into 2and3.

* correct platform name

euresti/typeshed,azag0/typeshed,li-dan/typeshed