text: Title: pyproject.toml uses non-public __legacy__  setuptools backend
  Body: * gevent version: git (as of beb98ce4742181caaa6cec9cedecaaa2a04f910b)
  * Python version: n/a
  * Operating System: Gentoo Linux
  
  ### Description:
  
  Per https://github.com/pypa/setuptools/issues/1689:
  
  > `setuptools.build_meta:__legacy__` was intended _only_ as a default and was not intended to be specified as the build backend.
  
  Please fix the build system to work correctly with the `setuptools.build_meta`. The explicit use of legacy backend is no longer supported on Gentoo and prevents us from packaging the new gevent version.