gh-150579: use lazy imports for concurrent.futures (GH-150585)

gh-150579: use lazy imports for concurrent.futures (GH-150585)

This module has a manual lazy import hack using `__getattr__`. Now that lazy imports exist and cannot be disabled, this could use lazy imports instead.

Key differences: this will now show up in sys.lazy_modules when accessed. Error messages should be a bit better without the wrapper `__getattr__` involved.  That's the only differences I can think of.

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>
Co-authored-by: Gregory P. Smith <greg@krypto.org>