Partial migration to support Slim 3 (and full phpunit 9 migration)

[Patabugen]

I started migrating SlimControllers for a project of mine, before realising the project had too many other dependencies so I've stopped the attempt.

I thought I'd publish this in case it's useful to anyone else, or anyone would like to finish it off.

This is not backwards compatible - this version would not work with Slim2.

I've converted all the tests to run on phpunit 9 (largly thanks to the help of [rector/rector]) and have fixed about half of the tests.

I've not made any real progress on the Middleware or ServiceController tests.

[ostark]

Is slim still a thing? this package is fairly unmaintained.

[Patabugen]

SlimPHP itself is still going strong - though they're on Version 4 and my legacy project is using 2.