Update instructions to install and activate using poetry

[pradeep90]

Hypothesis tests will fail on a clean checkout unless you install the extra dependencies.

Also, poetry shell no longer works.

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

[sobolevn]

👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (82ef3ef) to head (6bca296).
Report is 344 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #2064    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           80        81     +1     
  Lines         2485      2576    +91     
  Branches       437        44   -393     
==========================================
+ Hits          2485      2576    +91     

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