There are type annotations that indirectly depend on the
typing_extensions module (on Python 3.5-3.7: Protocol was added to the
core typing module in 3.8). The typing_extensions module is not
installed by default, so the code didn't run on a pristine Python
installation.
To avoid depending on a non-default module, make the dependency on
typing_extensions optional. (It's still required to run mypy, but
installing mypy takes care of providing typing_extensions.) If it
isn't available, provide a substitute definition that's just good
enough to get the scripts to run.
Move this ugly code to its own module to avoid the temptation of
spreading such ugliness all over the place. It's likely to be used in
other modules anyway.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
