[RFC] React 17

[eps1lon]

What would you like improved?

Support react 17.

1. widen peer dependency range or change it to require 17
2. test with 17

Since enzyme needs some time with React 17 (enzymejs/enzyme#2430) it might be worth investigating if @testing-library/react is sufficient for testing in this repository. I haven't looked closely since I wanted to get maintainer input before proposing such a big change to the repo internal.

[eps1lon]

Just realized that >=16.6.0 is specified so 17.0 already matches. Still filed a PR that adds a testing setup for 16 and 17 so that we know we're safe.