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

pyproject.toml

@@ -1,6 +1,5 b''
 [build-system]
 requires = ["setuptools", "wheel"]
-build-backend = "setuptools.build_meta"
 
 [tool.black]
 line-length = 80