enable CronWorkflow.from_dict() to work with subclasses

[crflynn]

Pull Request Checklist

• Fixes #
• Tests added
• Documentation/examples added
• Good commit messages and/or PR title

Description of PR
Currently, when subclassing CronWorkflow, the from_dict method returns an instance of CronWorkflow rather than the subclass.

This PR changes the creation of the instance to use cls rather than CronWorkflow, so that from_dict returns an instance of the subclass.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (d553546) to head (193d535).
Report is 133 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1039     +/-   ##
=======================================
+ Coverage   81.7%   81.8%   +0.1%     
=======================================
  Files         54      56      +2     
  Lines       4208    4228     +20     
  Branches     889     892      +3     
=======================================
+ Hits        3439    3461     +22     
+ Misses       574     571      -3     
- Partials     195     196      +1     

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[elliotgunton]

LGTM, thanks!