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

testing/test_plugin.py

@@ -12,7 +12,8 @@ import pytest
 @pytest.fixture
 def monkeypatch_3_cpus(monkeypatch: pytest.MonkeyPatch):
     """Make pytest-xdist believe the system has 3 CPUs"""
-    monkeypatch.setitem(sys.modules, "psutil", None)  # block import
+    # block import
+    monkeypatch.setitem(sys.modules, "psutil", None)  # type: ignore
     monkeypatch.delattr(os, "sched_getaffinity", raising=False)
     monkeypatch.setattr(os, "cpu_count", lambda: 3)