chore: bump version to 4.2.6a1

[psychedelicious]

Summary

Bump version to 4.2.6a1.

• The last released version was 4.2.5, but it had issues.
• The last version tag was 4.2.5post1. This included the MPS black images fixes, but was never released because we had reports of other memory issues.
• To make the ordering of versions work with pypi, the next version will be 4.2.6. We are skipping 4.2.5 as a stable release.

Related Issues / Discussions

n/a

QA Instructions

n/a

Merge Plan

I will do the release workflow after merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)