pep-517: remove the `build-backend` key to allow for users to build extensions PEP 517 changed how projects should define their build dependencies. The presence of a `pyproject.toml` file changes the behavior of `pip` to conform to PEP 517. Since we haven't updated the `setup.py` file yet (I'm not even sure we're not an edge case which would make it harder/impossible, I have not tried yet), this is a workaround to issue6589: it allows users to pass `--no-use-pep517` to pip to revert to the legacy installer. The build backend is simply assumed to be the `build_meta:__legacy__` one. Differential Revision: https://phab.mercurial-scm.org/D11393