Fastapi: Use `>=` instead of `==` for the black requierement in the dev-dependencies

[a-recknagel]

What?

The title says it all.

Why?

I'm using fastapi[tests] in my dev-dependencies, and would like to update to newer black versions as soon as they are released. Since fastapi uses == for black versions, that's not possible.

How?

It's very simple: #2249, I also included an update to the current 20.* version since it's been out for a while now, and ran it over fastapi/, pending_tests/, scripts/, and tests/.

[ArcLightSlavik]

fastapi[test] are meant for tests in the library itself.
If your making your own tests you need to use fastapi[all] which doesn't install black.

[a-recknagel]

Ah, that solves it. Thanks for the info!