Require paramtools 0.20.0 or higher

[martinholmer]

This pull request should pass all the tests on GitHub as soon as ParamTools PR #146 is merged and a new paramtools package is made available.

This PR resolves the problem reported in issue #2892.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9f3d863) to head (c7588c7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2905   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         2618      2618           
=========================================
  Hits          2618      2618           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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