Fix tox so that it can run

[kurtmckee]

This PR introduces the following changes:

• It updates the tox config so that it can run.

  The setup.py file is gone, as are the requirements.txt files. Poetry is invoked to install the dependencies now.

• It updates the list of Python versions to run the test suite against.

• It removes the now-unused default and simplejson factors

Taken together, these changes allow developers to run the test suite locally before submitting PRs to the project.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.09%. Comparing base (97e44a9) to head (5211ff3). Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #408   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files          24       24           
  Lines         759      759           
  Branches       80       80           
=======================================
  Hits          699      699           
  Misses         42       42           
  Partials       18       18           

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[kurtmckee]

Rebased on master

[kurtmckee]

@p1c2u I hope you're doing well! I wanted to ping you and ask if you could review some of these open PRs?