prepare to deprecate python 3.5

[briantist]

SUMMARY

Related: #81 & #82

Will rebase after #106 to re-apply commits and update the new requirements.txt for integration tests.

• Update requirements and constraints in accordance with the recent info from hvac around deprecating py3.5 at the same time as py2
• Update readme to indicate py3.5 deprecation
• Add changelog fragment to ensure py3.5 deprecation gets into porting guide

ISSUE TYPE

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ADDITIONAL INFORMATION

[codecov]

Codecov Report

Merging #107 (b6c44b7) into main (45aae79) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #107   +/-   ##
=======================================
  Coverage   83.52%   83.52%           
=======================================
  Files          16       16           
  Lines         892      892           
  Branches       87       87           
=======================================
  Hits          745      745           
  Misses        129      129           
  Partials       18       18           

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