`python_implementation` marker used in `build-system.requires` is non-standard and unsupported by alternative installers

### Describe the bug



SQLAlchemy declares a conditional build dependency on Cython using the `python_implementation` environment marker. This is a non-standard, undocumented marker, which the [packaging](https://github.com/pypa/packaging/issues/72) library used by pip has inherited from setuptools, but which alternative installers might not support. [uv](https://github.com/astral-sh/uv) does not support this marker. Please consider using `platform_python_implementation` or `implementation_name` instead.  For a list of standard markers, see https://packaging.python.org/en/latest/specifications/dependency-specifiers/#environment-markers.



### Optional link from https://docs.sqlalchemy.org which documents the behavior that is expected



_No response_



### SQLAlchemy Version in Use



2.1.0b1.dev0



### DBAPI (i.e. the database driver)



n/a



### Database Vendor and Major Version



n/a



### Python Version



any



### Operating system



any



### To Reproduce





Attempt installing SQLAlchemy from source with [uv](https://github.com/astral-sh/uv).







### Error



Installer-dependent.



### Additional context



_No response_

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