fix: improve str exceptions and consistency with python

[Skylion007]

Description

• Use error_already_set() to provide more informative exceptions and better alignment with CPython exception strings
• Replaces the related compatibility macro.
• Raise CodecError instead of TypeError in the byte constructor to be consistent with the fix in Raise codec errors when casting to std::string #2903

Suggested changelog entry:

* Improve exception handling in python str bindings

[jagerman]

Revert macro change just in case

Why keep the macro?

[Skylion007]

Why keep the macro?

In case some third party caster uses it? I am fine with nuking them, but we should probably make a separate PR to nuke them all at once.

[henryiii]

I see at least a few users:

https://cs.github.com/?scopeName=All+repos&scope=&q=PYBIND11_BYTES_AS_STRING_AND_SIZE

Tencent/MMKV: POSIX/Python/libmmkv_python.cpp‎ for example.

[henryiii]

(There are quite a lot of users of our compatibility macros overall - if we want to get rid of them, we need deprecation messages, etc.)

[jagerman]

Hurray for downstream users using undocumented internal macros! 😞

[rwgk]

MUCH better, thanks for cleaning this up Aaron!