Move stubs for 'click' into 2and3 folder

[Michael0x2a]

The "click" 3rd party library is currently listed as being Python 3.6+ only on typeshed.

The exact Python versions click supports is apparently unclear, but it definitely includes Python 2.7 and recent versions of Python 3, so moving it into the 2and3 folder is probably best.

[ambv]

Originally they were put in 3.6 because mypy didn't accept variable annotations for 3.5 and older stubs, and we wanted to test if those actually work. So moving to 3/ is not a big deal at all.

Moving to 2and3 is maybe going to give you problems with strings because I just used "str" everywhere.

So, I'm okay with the change but beware that you're likely going to have some cleaning up to do.