[docs] Add explicit support for python 3.7

[villebro]

CATEGORY

Choose one

• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

Currently the PyPI package only officially supports 3.6, and the documentation specifies that "running on 3.7 should work". As I've now been running 3.7 exclusively in dev for at least 6 months without any trouble, committing to officially supporting 3.7 should be safe.

REVIEWERS

@john-bodley @mistercrunch

[codecov-io]

Codecov Report

Merging #8309 into master will decrease coverage by 5.79%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8309      +/-   ##
==========================================
- Coverage   73.53%   67.74%    -5.8%     
==========================================
  Files         115      451     +336     
  Lines       12396    22699   +10303     
  Branches        0     2370    +2370     
==========================================
+ Hits         9116    15378    +6262     
- Misses       3280     7184    +3904     
- Partials        0      137     +137

Impacted Files	Coverage Δ
...sets/src/explore/components/ExploreChartHeader.jsx	50% <0%> (ø)
superset/assets/src/components/Checkbox.jsx	100% <0%> (ø)
...perset/assets/src/dashboard/util/getEmptyLayout.js	0% <0%> (ø)
...ts/src/dashboard/util/getComponentWidthFromDrop.js	95.23% <0%> (ø)
.../assets/src/dashboard/reducers/dashboardFilters.js	88.23% <0%> (ø)
...rc/explore/components/controls/AnnotationLayer.jsx	2.46% <0%> (ø)
superset/assets/src/welcome/DashboardTable.jsx	87.5% <0%> (ø)
...erset/assets/src/dashboard/util/injectCustomCss.js	91.66% <0%> (ø)
...explore/components/controls/ColorPickerControl.jsx	85% <0%> (ø)
...ets/src/dashboard/components/dnd/DragDroppable.jsx	94.59% <0%> (ø)
... and 326 more

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