3.7.1 release branch (plotly#1470)

* CHANGELOG
* README version updates
* No change needed to plotlywidget extension

Update README installation versions

Update README installation versions

Release 3.6.1 (plotly#1430)

* Update to plotly.js 1.44.3
* 3.6.1 changelog

Release 3.6.1 (plotly#1430)

* Update to plotly.js 1.44.3
* 3.6.1 changelog

Release 3.6.0 (plotly#1412)

* Update to plotly.js 1.44.1
* 3.6.0 Changelog Entry
* Update README with 3.6.0 instructions

Release 3.5.0 (plotly#1387)

Release 3.4.2 (plotly#1285)

* Updated changelog for 3.4.2
* Updated README installation instructions
* Update versions to plotly.py 3.4.2 and plotlywidget 0.5.2

Release 3.4.1 (plotly#1269)

* quiet pip install in conda packaging to see if it helps avoid circleci build errorerror: [Errno 11] Resource temporarily unavailable.   Running setup.py install for plotly: finished with status 'error'
* Update Plotly.js to 1.42.5
* Update CHANGELOG with new fixes and new plotly.js version
* Bump plotly.py version to 3.4.1 and plotlywidget to 0.5.1

Release 3.4.0 (plotly#1254)

* Added changelog and README version updates for 3.4.0
* Update plotly.js to 1.42.2
* Darken default marker outline color in plotly_dark template
* Fix ggplot2 colorbar length
* Bump plotly.py and plotlywidget versions to 3.4.0 and 0.5.0