Require python 3.6 and CI maintenance (#3160)

* Update setup.py to require Python>=3.6
* Add an f-string (compatible with python>=3.6) for good measure
* Remove tox
* Rename plot_ly in chart_studio in ci tests
* Move tests with pandas/numpy dependencies to test_optional, remove these from core requirements
* Move dependency check tests to optional
* Don't open browser window in matplotlylib test
* Add Python 3.8 and 3.9 tests

Author: jonmmease

test_requirements/requirements_37.txt

@@ -0,0 +1,12 @@
+decorator==4.0.9
+nose==1.3.7
+requests==2.12.4
+six==1.10.0
+pytz==2016.10
+retrying==1.3.3
+pytest==3.5.1
+pandas==0.23.2
+numpy==1.14.3
+ipywidgets==7.2.0
+matplotlib==2.2.3
+--editable=./plotly