compat: fallback to toml if tomli is not available

[FFY00]

We already have the code, as we support Python < 3.6, and being able to
fallback to toml if tomli is not available will make bootstraping Python
environments much easier.

Signed-off-by: Filipe Laíns lains@riseup.net

[takluyver]

I've actually been working on documenting how to bootstrap things without relying on setuptools: pypa/flit#441

As things stand, I'd be OK with doing the fallback like this. But I don't want to imply that the recommended way to bootstrap these packages is using setuptools. At some point - possibly quite soon - we'll drop support for Python 3.6, which is already out of security updates from the core developers. When we do, I'd like to go back to just importing one TOML parser (tomli). So bootstrappers should be prepared to do things that way.

[FFY00]

Once we get the wheel installer ready, I plan to write a bootstraping script that can build from source, and install, the packages necessary to bootstrap a Python environment, which should fix that issue.

Regardless of the future handling of the TOML parser dependency on this project, simply having a release that works with both toml and tomli would still be very helpful. So, if you could merge this and tag a release, it would be great. After that, feel free to hard depend on tomli.

[takluyver]

Closing following discussion on #131 - @FFY00 said this turns out not to be so useful after all.