What's Changed
* Bump backend version with data race fix by MilesCranmer in https://github.com/MilesCranmer/PySR/pull/268
* Incorporates depth check into constraints, rather than in mutation step.
* Fixes one instance of a data race (appears to be remaining issues, however)
**Full Changelog**: https://github.com/MilesCranmer/PySR/compare/v0.11.14...v0.11.15