🔇 remove the warning as it might confuse non dash usage

[byronz]

this PR will fix one part of #946 and added missing python_requires in setup.py

[alexcjohnson]

Wouldn't we also need to put all the below definitions into the try block, so that without the necessary imports it won't try to define our fixtures?

That said, this still doesn't seem like it completely satisfies the request of #946 - this takes care of the "annoys with the warning" part, but not the "increases the time of test suite by unnecessary import of the module" part. As I said in the issue, it's not clear to me how one can resolve that part, but the ideal seems to me determining whether setup.py was called with [testing] or not - and only include "pytest11": ["dash = dash.testing.plugin"] if yes.

[alexcjohnson]

Alright, this is a definite improvement - as long as we leave #946 open and keep our eyes out for a more complete solution, I'm happy to add this piece 💃