Add build-system to pyproject.toml

So doesn't use legacy stuff.

Fix #719.

changelog/719.trivial.rst

@@ -0,0 +1 @@
+Use up-to-date ``setup.cfg``/``pyproject.toml`` packaging setup.

pyproject.toml

@@ -1,3 +1,15 @@
+[build-system]
+requires = [
+  # sync with setup.py until we discard non-pep-517/518
+  "setuptools>=45.0",
+  "setuptools-scm[toml]>=6.2.3",
+  "wheel",
+]
+build-backend = "setuptools.build_meta"
+
+[tool.setuptools_scm]
+write_to = "src/xdist/_version.py"
+
 [tool.towncrier]
 package = "xdist"
 filename = "CHANGELOG.rst"

setup.cfg

@@ -40,7 +40,7 @@ install_requires =
     execnet>=1.1
     pytest>=6.0.0
     pytest-forked
-setup_requires = setuptools_scm
+setup_requires = setuptools_scm>=6.0
 
 [options.packages.find]
 where = src

setup.py

@@ -1,6 +1,4 @@
 from setuptools import setup
 
 if __name__ == "__main__":
-    setup(
-        use_scm_version={"write_to": "src/xdist/_version.py"},
-    )
+    setup()