The use of -S ensures that only the CPython standard library is accessible,
which makes tests run the same regardless of any site-packages that are
installed. It also improves start-up time of CPython, reducing the overall
time spent running the test suite.
tests/basics/containment.py is updated to work around issue with old Python
versions not being able to str-format a dict-keys object, which becomes
apparent when -S is used.
Signed-off-by: Damien George <damien@micropython.org>
