Scala 3 (Dotty) support

[larsrh]

Collaborative PR: @alexandru @Avasil @larsrh

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Prequel PRs:

• Scala 2/3 compatiblity #1326
• deprecate Scheduler extension methods, move them to Scheduler #1327
• deprecate SchedulerService extension methods #1328
• add missing method implementation to JSArrayQueue #1329
• more Scala 2/3 compatiblity #1331
• Dotty compatibility #1344

[larsrh]

There a still four compile errors in the doctest reactiveJVM:

[error] -- Error: /home/lars/proj/monix/monix/monix-reactive/jvm/target/scala-3.0.0-M1/src_managed/test/monix/reactive/ObservableDoctest.scala:446:11 
[error] 446 |    ).merge
[error]     |           ^
[error]     |no implicit argument of type monix.reactive.OverflowStrategy[B] was found for parameter os of method merge in class Observable

I find that strange; I thought the type ascription to the implicit argument of merge fixed these.

[Avasil]

There a still four compile errors in the doctest reactiveJVM:

[error] -- Error: /home/lars/proj/monix/monix/monix-reactive/jvm/target/scala-3.0.0-M1/src_managed/test/monix/reactive/ObservableDoctest.scala:446:11 
[error] 446 |    ).merge
[error]     |           ^
[error]     |no implicit argument of type monix.reactive.OverflowStrategy[B] was found for parameter os of method merge in class Observable

I find that strange; I thought the type ascription to the implicit argument of merge fixed these.

I've opened an issue about it scala/scala3#10497

[larsrh]

I've opened an issue about it

Ah right, I forgot that existed 😁

[Avasil]

@larsrh I have finally spent some time on it and the good news is that almost all tests pass! I think the only issue left is Observable#merge...

[alexandru]

I solved merge by fixing the tests 🤷‍♂️ as there's nothing we can do for now, the only other options being:

1. to wait for that bug to get fixed in Scala 3, or ...
2. break compatibility

We can break compatibility in 4.0.0, but for now this will have to do.

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I also upgraded sbt, and cleaned up all warnings, except for those weird ones that aren't reported as actual warnings and that may be the result of some plugin.

Not sure if I want to do anything else before merging.

[larsrh]

Please hold back with merging until I get the chance to do one final review pass, okay?

[alexandru]

@larsrh sounds good, thanks!

[oleg-py]

Looks good to me

[larsrh]

I noted a few issues where there are inconsistencies between Scala 2 and 3 and (possibly?) unrelated changes that I don't understand. But those aren't blockers. Good work, everyone 💪