_context.py: Cache result of get_strict()

While get_strict() doesn't look expensive per-se, it is called so many
times that it is valuable to cache the result once computed.  Since I
don't think it can change once it is computable, cache it immediately
that becomes possible and we save 20s in my test case.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>