Update the README to drop Python 3.5 support

[brettcannon]

isort 5 dropped Python 3.5 and it also reaches EOL on September 13, 2020 which is less than a month away.

Closes #13459.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR).
• Title summarizes what is changing.
• Has a news entry file (remember to thank yourself!).
• Appropriate comments and documentation strings in the code.
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated.
• Test plan is updated as appropriate.
• package-lock.json has been regenerated by running npm install (if dependencies have changed).
• The wiki is updated with any design decisions/details.

[codecov-commenter]

Codecov Report

Merging #13505 into master will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13505      +/-   ##
==========================================
- Coverage   59.86%   59.75%   -0.11%     
==========================================
  Files         670      670              
  Lines       37295    37363      +68     
  Branches     5335     5357      +22     
==========================================
+ Hits        22326    22328       +2     
- Misses      13815    13877      +62     
- Partials     1154     1158       +4     

Impacted Files	Coverage Δ
src/client/pythonEnvironments/info/index.ts	41.53% <0.00%> (-27.43%)	⬇️
src/client/common/utils/platform.ts	64.70% <0.00%> (-11.77%)	⬇️
src/client/datascience/crossProcessLock.ts	79.41% <0.00%> (-11.77%)	⬇️
src/client/datascience/jupyter/kernels/helpers.ts	35.51% <0.00%> (-10.27%)	⬇️
...rc/client/pythonEnvironments/info/pythonVersion.ts	87.87% <0.00%> (-8.68%)	⬇️
src/client/linters/pydocstyle.ts	86.66% <0.00%> (-2.23%)	⬇️
src/client/datascience/debugLocationTracker.ts	76.56% <0.00%> (-1.57%)	⬇️
src/client/common/process/proc.ts	14.49% <0.00%> (-0.73%)	⬇️
src/client/datascience/constants.ts	99.78% <0.00%> (+<0.01%)	⬆️
...ient/datascience/jupyter/kernels/kernelSelector.ts	63.95% <0.00%> (+0.04%)	⬆️

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[sonarcloud]

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