fix and update packaging

* drop empty setup.py which "breaks" on incomplete build envs
* use the build package to build the dist
* update the gh workflow to use it

fixup noise from pre-commit

src/xdist/plugin.py

@@ -18,7 +18,9 @@ def pytest_xdist_auto_num_workers(config):
         try:
             return int(env_var)
         except ValueError:
-            warnings.warn("PYTEST_XDIST_AUTO_NUM_WORKERS is not a number: {env_var!r}. Ignoring it.")
+            warnings.warn(
+                "PYTEST_XDIST_AUTO_NUM_WORKERS is not a number: {env_var!r}. Ignoring it."
+            )
 
     try:
         import psutil