Add stub pyproject.toml file

This keeps our setup.py around for now, while still supporting a PEP-517
compliant build. It's the minimum required change to make modern pyroma stop
complaining, and hopefully to avoid deprecation warnings.

Partially duplicates #8560
See #3526

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools>=42"]
+build-backend = "setuptools.build_meta"

setup.py

@@ -10,6 +10,14 @@ import re
 import ast
 import os
 import os.path
+import sys
+
+# Add repo root to path so we can import scripts. Prior to PEP517 support this
+# was the default behavior for setuptools.
+# https://github.com/pypa/setuptools/issues/3939#issuecomment-1573619382
+# > If users want to import local modules they are recommended to explicitly add
+# > the current directory to sys.path at the top of setup.py.
+sys.path.append(".")
 
 from scripts import setupcommon as common