Obtain the original locale in the documented way.  This way actually
works for me.

Restore the original locale at the end, instead of forcing to "C".

Move the locale fiddling into the test driver instead of doing it as a
side effect of merely importing the module.  I don't know why the test
is mucking with locale (and also added a comment saying so), but it
surely has no justification for doing that as an import side-effect.
Now whenever the locale-changing code executes, the locale-restoring code
will also get run.