case-sensitive filesystems -- which is not the default case. Along the way also
fixed the skipping of tests when sys.dont_write_bytecode is true.
Closes issue #5442 again.
(mostly stuff specified by PEP 302). There are two ABCs, PyLoader and
PyPycLoader, which help with implementing source and source/bytecode loaders by
implementing load_module in terms of other methods. This removes a lot of
gritty details loaders typically have to worry about.
Brett Cannon