Add RxJava3 version

[PattaFeuFeu]

Adds an RxJava3-specific version alongside those for RxJava(1) and RxJava2.

RxJava2 has been in maintenance mode since February this year and, while compatibility layers are available, it’s still far easier and better to use updated libraries which are based on RxJava3 already.

The main change is in the imports used by all the classes. But since some methods were removed, the test class had to be slightly changed. TestSubscriber#assertNotTerminated has been removed and is therefore replaced by TestSubscriber#assertNoErrors().assertNotComplete(). Also, TestSubscriber#dispose now doesn’t allow access outside of the package, so it’s replaced by TestSubscriber#cancel (which is called by dispose anyway).