pyproject.toml: specify build-system requirements

Using pyproject.toml to specify build system requirements is now the
preferred way. From the setuptools documentation[1]:

"This used to be accomplished with the setup_requires keyword but is now
considered deprecated in favor of the PEP 517 style described above."

I didn't remove the 'setup_requires' from setup.py because some users
who are using older versions of setuptools might still rely on it if
their version of setuptools has no support for pyproject.toml.

Without this pyproject.toml, the following command fails on a local
network without internet access:

  pip install --index-url <local index> -e .

because setuptools does not take the index into account when fetching
the 'setup_requires' dependencies defined in setup.py.

However, with the correct pyproject.toml available, the build system
requirements are fetched first, using the correct index, and then later
at the point where setuptools tries to install the 'setup_requires'
dependencies, they are already installed so no need to fetch anything
anymore.

[1]: https://setuptools.pypa.io/en/latest/userguide/dependency_management.html

Signed-off-by: Sam Van Den Berge <sam.van.den.berge@gmail.com>

Author: S4mw1s3

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+# Minimum requirements for the build system to execute.
+requires = ["setuptools", "wheel", "setuptools_scm"]