Defer `pytest` import?

[chriddyp]

Looks like pytest isn't the safest dependency, causing issues with scikit-learn: https://community.plot.ly/t/pytest-transient-dependency/25383

Could we move the import pytest into the testing module/class/function itself and not require it upon install? We could even have a separate install with setup.py's "extras" feature (https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies) like pip install dash[testing] or something.

[byronz]

@chriddyp in 1.0, we already use the extras feature in setup.py, so the pip install dash[testing] might be the way to go.