Build pip-tools using PyPA's build frontend tool

[otherJL0]

What's the problem this feature will solve?

Running the current build command:

python3 setup.py sdist --format=gztar bdist_wheel

produces the following warning:

/opt/homebrew/lib/python3.8/site-packages/setuptools/installer.py:27: 
SetuptoolsDeprecationWarning: setuptools.installer is deprecated. 
Requirements should be satisfied by a PEP 517 installer.

The setuptools_scm repo indicates that using setup.py is deprecated in favor of using pyproject.toml.
https://github.com/pypa/setuptools_scm#setuppy-usage-deprecated

Describe the solution you'd like

Since the project's setup.py file is only populated to target setuptools_scm, migrating
to an equally sparse pyproject.toml is a simple task:

# Current setup.py
from setuptools import setup

setup(use_scm_version=True)

# Equivalent pyproject.toml
[tool.setuptools_scm]

If the project migrates away from using setup.py, a (hopefully) painless alternative is to use
build, which is already part of pip-tool's dependencies'.
To use build, the proposed pyproject.toml needs to indicate the proposed build backend:

# Updated proposed pyproject.toml
[build-system]
requires = ["setuptools>=45", "setuptools_scm[toml]>=6.2", "wheel"]
build-backend = "setuptools.build_meta"

[tool.setuptools_scm]

With the proposed pyproject.toml file pip-tools can now be built using

python -m build # entry point
pyproject-build # equivalent console script

According to the official docs, using build is roughly equivalent to running the current build step:

setup.py sdist bdist_wheel

https://pypa-build.readthedocs.io/en/latest/differences.html#setup-py-sdist-bdist-wheel

Additional context

setuptools_scm repository
build repository
build docs