bpo-44002: Switch to lru_cache in urllib.parse. (GH-25798)

bpo-44002: Switch to lru_cache in urllib.parse. (GH-25798)

Switch to lru_cache in urllib.parse.

urllib.parse now uses functool.lru_cache for its internal URL splitting and
quoting caches instead of rolling its own like its the 90s.

The undocumented internal Quoted class API is now deprecated
as it had no reason to be public and no existing OSS users were found.

The clear_cache() API remains undocumented but gets an explicit test as it
is used in a few projects' (twisted, gevent) tests as well as our own regrtest.