* Remove documentation link to TimezoneTransition (removed in #958)

* Document more APIs

* Upgrade Sphinx

* Belatedly bump version in docs

Currently `_cache` is only populated on `load`, this is an issue with
code which do a lot of locale parsing / instantiation but for one
reason or an other rarely end up needing to actually load the locale
data (e.g. date formatting with non-locale-dependent patterns) because
every cache miss is an `os.path.exists`.
Cache `exists` separately.

Update test because `lru_cache` requires all parameters to be
hashable and a list is not that.

Also implemented fallback from the correct pluralization to "other", which was also missing.

Fixes #1217.