[docs] suggest using python3.6 in installation docs

[mistercrunch]

No description provided.

[codecov-io]

Codecov Report

Merging #5834 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5834      +/-   ##
==========================================
- Coverage    63.7%   63.68%   -0.03%     
==========================================
  Files         366      368       +2     
  Lines       23146    23187      +41     
  Branches     2599     2600       +1     
==========================================
+ Hits        14746    14767      +21     
- Misses       8385     8405      +20     
  Partials       15       15

Impacted Files	Coverage Δ
...ts/src/dashboard/components/gridComponents/Tab.jsx	82.43% <0%> (-3.87%)	⬇️
superset/assets/src/explore/components/Control.jsx	38.09% <0%> (ø)	⬆️
superset/assets/src/visualizations/filter_box.jsx	0% <0%> (ø)	⬆️
...uperset/assets/src/visualizations/paired_ttest.jsx
superset/assets/src/visualizations/BigNumber.jsx
.../assets/src/visualizations/BigNumber/BigNumber.jsx	0% <0%> (ø)
.../src/dashboard/components/DeleteComponentModal.jsx	86.95% <0%> (ø)
...sets/src/visualizations/PairedTTest/TTestTable.jsx	0% <0%> (ø)
...ets/src/visualizations/PairedTTest/PairedTTest.jsx	0% <0%> (ø)
superset/assets/src/components/ModalTrigger.jsx	97.22% <0%> (+0.16%)	⬆️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5616d7b...7f6a656. Read the comment docs.