Add site.alleles method

[hyanwong]

Description

Deliberately returns an (unordered) set

Fixes #2380

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[codecov]

Codecov Report

Merging #2385 (a847133) into main (845ccec) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head a847133 differs from pull request most recent head 62d70f4. Consider uploading reports for the commit 62d70f4 to get more accurate results

@@           Coverage Diff           @@
##             main    #2385   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files          28       28           
  Lines       26917    26921    +4     
  Branches     1239     1240    +1     
=======================================
+ Hits        25111    25115    +4     
  Misses       1772     1772           
  Partials       34       34           

Flag	Coverage Δ
c-tests	92.25% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	70.99% <25.00%> (-0.02%)	⬇️
python-tests	98.81% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
python/tskit/trees.py	98.16% <100.00%> (+<0.01%)	⬆️

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[jeromekelleher]

Looks good, some suggeestions

[benjeffery]

Can we add a test where the ancestral state is not a single character? Easy to trip up when python does things like this:

>>> a = "foobar"
>>> set(a)
{'f', 'o', 'b', 'a', 'r'}
>>> {a}
{'foobar'}

[hyanwong]

Comments addressed. Thanks @jeromekelleher and @benjeffery. Ready to merge I think.