Remove `setup.py`

Fix #11667.

changelog/11667.breaking.rst

@@ -0,0 +1,3 @@
+pytest's ``setup.py`` file is removed.
+If you relied on this file, e.g. to install pytest using ``setup.py install``,
+please see `Why you shouldn't invoke setup.py directly <https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html#summary>`_ for alternatives.

pyproject.toml

@@ -1,6 +1,5 @@
 [build-system]
 requires = [
-  # sync with setup.py until we discard non-pep-517/518
   "setuptools>=45.0",
   "setuptools-scm[toml]>=6.2.3",
 ]

setup.py

@@ -1,4 +0,0 @@
-from setuptools import setup
-
-if __name__ == "__main__":
-    setup()