add post_order

[rtimms]

Description

Add post_order method to symbol to return an iterable that steps through the tree in post-order fashion

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python -m pytest (or $ nox -s tests)
• The documentation builds: $ python -m pytest --doctest-plus src (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[martinjrobins]

LGTM, thanks @rtimms

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (864388f) to head (0b257a6).
Report is 146 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4684   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          302      302           
  Lines        22880    22883    +3     
========================================
+ Hits         22701    22704    +3     
  Misses         179      179           

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