bpo-43012: remove `pathlib._Accessor` (GH-25701)

Per Pitrou:

> The original intent for the “accessor” thing was to have a variant that did all accesses under a filesystem tree in a race condition-free way using openat and friends. It turned out to be much too hairy to actually implement, so was entirely abandoned, but the accessor abstraction was left there.

https://discuss.python.org/t/make-pathlib-extensible/3428/2

Accessors are:

- Lacking any internal purpose - '_NormalAccessor' is the only implementation
- Lacking any firm conceptual difference to `Path` objects themselves (inc. subclasses)
- Non-public, i.e. underscore prefixed - '_Accessor' and '_NormalAccessor' 
- Unofficially used to implement customized `Path` objects, but once once [bpo-24132]() is addressed there will be a supported route for that.

This patch preserves all existing behaviour.