V4 refine dependencies

[jonmmease]

This PR cleans up the distribution package dependencies for the plotly package now that chart_studio has been split off.

I was able to remove 3 hard dependencies:

1. I replaced the nbformat hard-dependency with runtime validation (this fully resolves unneeded dependency on nbconvert #771). nbformat is only needed for mimetype rendering, so that's where the validation happens. It's not needed to generate HTML output.
2. The pytz library is no longer needed now that we stopped performing time-zone translation (Stop auto-converting timezones to UTC and stripping timezone info #1581)
3. The requests library is only used for orca image export. There was already validation to make sure the optional psutil library is installed for use with orca, so I added requests validation there too.
4. the decorator package was only being used for an unused memoize function. So both the function and dependency have been removed.

Required dependencies for the plotly package are now: install_requires=["retrying>=1.3.3", "six"],