Run draft-next tests on PR submission

[handrews]

This only make sense if PR #97 is accepted (and even then it might not be what's wanted, which is why it's a separate PR).

This leaves the general tox.ini default in place, but runs the draft-next tests for PR submissions to incorporate their coverage (e.g. for PR #93). We could also enable the optional and format tests, I just didn't have a clear use case for that yet.

[handrews]

The tests will fail here because I didn't stack this on #97.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.14 🎉

Comparison is base (3ff123b) 92.62% compared to head (00636ed) 92.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   92.62%   92.76%   +0.14%     
==========================================
  Files          23       23              
  Lines        2033     2033              
  Branches      428      428              
==========================================
+ Hits         1883     1886       +3     
+ Misses         97       94       -3     
  Partials       53       53              

see 2 files with indirect coverage changes

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[handrews]

This now ought to pass. Still an open question as to whether this is the ideal behavior or not, but it would allow PR #80 [EDIT: I meant PR #93] to pass the coverage check (although that PR needs rewriting as noted in its move to "draft" status, so it's not an immediate blocker anyway).