http.cookies: parser accepts key characters that Morsel.set then rejects

Bug report

Bug description:

SimpleCookie.load() extracts cookie names using _LegalKeyChars, which deliberately admits characters RFC 6265 forbids in a token — the comment above it cites MSIE non-compliance as the reason. Morsel.set() then validates with _is_legal_key, built from _LegalChars, which excludes them. Twelve separator characters differ, so any Set-Cookie whose name contains one raises CookieError even though the module's own parser accepted it.

Encountered in the wild: F5 BIG-IP load balancers emit a persistence cookie named for the partition path, /Common/.app/--pool. Browsers accept this...

from http.cookies import SimpleCookie
SimpleCookie('/a/b=1')      # also a@b=1, a,b=1, a(b)=1

CPython versions tested on:

3.14

Operating systems tested on:

Linux

El issue menciona SimpleCookie.load(), Morsel.set(), _LegalKeyChars, _is_legal_key y _LegalChars; empieza siguiendo esas rutas del parser y de validación y reproduciendo los nombres de cookie indicados. Se considera terminado cuando el comportamiento de los nombres aceptados sea coherente, los ejemplos ya no generen CookieError y exista cobertura de regresión para la discrepancia.