Parallel filesystem cache for compiled bytecode files

The new :envvar:`PYTHONPYCACHEPREFIX` setting (also available as

:option:`-X` ``pycache_prefix``) configures the implicit bytecode

cache to use a separate parallel filesystem tree, rather than

the default ``__pycache__`` subdirectories within each source

directory.

The location of the cache is reported in :data:`sys.pycache_prefix`

(:const:`None` indicates the default location in ``__pycache__``

subdirectories).

(Contributed by Carl Meyer in :issue:`33499`.)