Most pure Python libraries run on PyPy without porting, while incompatibilities come from C extensions written against the CPython C-API such as numpy, lxml and many crypto libraries that either fail or run poorly under PyPy's cpyext compatibility layer.

If you plan to support PyPy, add it to your CI, prefer cffi or pure Python fallbacks over CPython C-API extensions, and be ready to rewrite or vendor performance-critical C extensions because cpyext is slow and incomplete and will waste your debugging time.