build: Remove uneeded buid requirements

[matthewfeickert]

• The build requirements are the necessary build tooling. pip is an installer front end and not a build system.

[matthewfeickert]

@WolfgangWaltenberger This is ready for review.

[matthewfeickert]

The CI is failing from

FAILED: <class 'smodels.experiment.exceptions.SModelSExperimentError'>,"Error 404: could not fetch 'https://smodels.github.io/database/official303' from server: 'Not Found'"

which is unrealted to this PR.

[matthewfeickert]

To clarify a bit, the build-system.requires are tools that get installed from PyPI into a temporary virtual environment that the build happens in which is then cleaned up and removed. So pip isn't something that should get added here (unless you had to intentionally downgrade pip, but at that point you have what amounts to a broken build and you should fix your build instead).

c.f. https://learn.scientific-python.org/development/guides/packaging-classic/#pep-517518-support-high-priority for more high level details or read PEP 517/518.

[WolfgangWaltenberger]

Removed the pip dependency further down in the code.