build: Migrate from setup.py to pyproject.toml

Using `setup.py` is deprecated and the new recommanded way is to declare
a `pyproject.toml` file (see PEP 517 [^1]).

This commit proposes to use setuptools to achieve that, because
setuptools is already used by yamllint, is standard and referenced by
the official Python packaging documentation [^2], and seems to be the
most lightweight solution. An alternative could have been to use Poetry,
see the dedicated pull request and discussion [^3].

For some period, the `setup.py` file will be kept (although almost
empty), to allow old tools versions to keep working.

Closes https://github.com/adrienverge/yamllint/issues/509.

[^1]: https://peps.python.org/pep-0517/
[^2]: https://packaging.python.org/en/latest/tutorials/installing-packages/
[^3]: https://github.com/adrienverge/yamllint/pull/557

setup.py

@@ -15,15 +15,6 @@
 
 from setuptools import setup
 
-from yamllint import (__author__, __license__,
-                      APP_NAME, APP_VERSION, APP_DESCRIPTION)
-
-
-setup(
-    name=APP_NAME,
-    version=APP_VERSION,
-    author=__author__,
-    description=APP_DESCRIPTION.split('\n')[0],
-    long_description=APP_DESCRIPTION,
-    license=__license__,
-)
+# This is only kept for backward-compatibility with older versions that don't
+# support new packaging standards (e.g. PEP 517 or PEP 660):
+setup()