Warn about incorrect version in build-system.requires

[konstin]

Maturin should warn if the version in the pyproject.toml build system requirements is incompatible with the one that is currently running (#1792). We only warn instead of erroring to allow e.g. testing a prerelease version of maturin with a project without having to edit the requires.

pyproject.toml:

[build-system]
requires = ["maturin==0.0.1"]
build-backend = "maturin"

Output:

⚠️  Warning: You specified maturin ==0.0.1 in pyproject.toml under `build-system.requires`, but the current maturin version is 1.2.3

Fixes #1792.

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[messense]

Thanks!