The `exit()` [1] function is not necessarily present in every Python
environment, as it's added by the `site` module. Also, this function is
"[...] useful for the interactive interpreter shell and should not be
used in programs"
For this reason, we replace all such occurrences with `sys.exit()` [2],
which is the canonical function to exit Python programs.
[1]: https://docs.python.org/3/library/constants.html#exit
[2]: https://docs.python.org/3/library/sys.html#sys.exit
Replace our reference to an Apple development certificate with a
Developer ID Application certificate. The former is not accepted during
the code notarization phase, whereas the latter is.
Update several references to First Look Media in the code, to better
reflect the current status, where Freedom of the Press Foundation has
taken over the stewardship of the project.
Fixes#343
