ci: Block setuptools version in windows compilation

In setuptools v74, the `distutils.msvc9compiler` module has been
removed. However, `cffi` still relies on it. Therefore, the generation
of the windows package fails.

By adding a `pyproject.toml` file and declaring the build backend, it
is possible to define the setuptools version used by the command
`python3 - build` which generates the windows wheel.

See: python-cffi/cffi#117
See: https://packaging.python.org/en/latest/guides/writing-pyproject-toml/

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools == 73.0.1"]
+build-backend = "setuptools.build_meta"