[lit] Add pyproject.toml and fix build w/ modern setuptools backend

[mgorny]

Summary

Add a pyproject.toml file that provides build system information
for PEP 517-compliant builders. While all the commonly used builders
provide fallback to running setup.py for backwards compatibility, this
ensures the best forward compatibility. It also provides a reliable way
of specifying the minimum required setuptools version. Effectively, it
will make it possible to remove setup.py in favor of purely
declarative configuration in the future, or even switch to a different
build system.

Update setup.py to explicitly add the current directory to sys.path
for importing lit. This is necessary, as the modern setuptools
backend does not guarantee that the current directory is present there.

[MaskRay]

oes the pyproject.toml work in the absence of setup.py? It seems to lack [project] [project.urls] etc? Shall we adopt hatch?

[MaskRay]

Ah, ok, build-backend = "setuptools.build_meta" seems sufficient.

[mgorny]

In D136976#3898379, @MaskRay wrote:

oes the pyproject.toml work in the absence of setup.py? It seems to lack [project] [project.urls] etc? Shall we adopt hatch?

This one doesn't. I'd personally prefer flit_core as that's literally the simplest build system for Python packages (and the only one that doesn't vendor ad ozen or more random packages) but right now, my goal was to aim for a minimal change.

llvm/utils/lit/pyproject.toml

+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"

llvm/utils/lit/setup.py

-import lit
 import os
+import sys
 
 from setuptools import setup, find_packages
 
 # setuptools expects to be invoked from within the directory of setup.py, but it
 # is nice to allow:
 #   python path/to/setup.py install
 # to work (for scripts, etc.)
 os.chdir(os.path.dirname(os.path.abspath(__file__)))
+sys.path.insert(0, ".")
+
+import lit
 
 with open("README.rst", "r", encoding="utf-8") as f:
     long_description = f.read()
 
 setup(
     name = "lit",
     version = lit.__version__,