Needs pyproject.toml to declare build-time dependency on cysignals

[mkoeppe]

cypari2 is affected by pypa/pip#6193

Even though dependencies are declared in setup.py, by the latest pip build isolation policies, it does not install cysignals before doing bdist_wheel.

[videlec]

@mkoeppe Could you have a quick look at #104?

[mkoeppe]

Looks great to me except for the typo that I noted.