fix: expose ak.behaviors.mixins names at top level

[agoose77]

Fixes #1834

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/agoose77-fix-mixins-top-level-names/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1835 (da4aa8e) into main (cf26ac0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/__init__.py	97.14% <100.00%> (ø)
src/awkward/contents/indexedarray.py	76.77% <0.00%> (-0.19%)	⬇️
src/awkward/contents/recordarray.py	83.94% <0.00%> (-0.19%)	⬇️
src/awkward/_util.py	82.15% <0.00%> (-0.18%)	⬇️
src/awkward/contents/indexedoptionarray.py	88.96% <0.00%> (-0.16%)	⬇️
src/awkward/_connect/avro.py	87.17% <0.00%> (-0.14%)	⬇️
src/awkward/types/_awkward_datashape_parser.py	47.72% <0.00%> (-0.01%)	⬇️
src/awkward/contents/content.py	77.31% <0.00%> (+0.10%)	⬆️

[jpivarski]

That does it!

Since this is a simple fix, I assume that you're not planning on adding anything more, and I'll merge this as-is.