remove support for setup.cfg only projects

Per the discussion in pypa#9945.

Signed-off-by: Filipe Laíns <lains@riseup.net>

news/10031.bugfix

@@ -0,0 +1,3 @@
+Remove support for ``setup.cfg``-only projects.
+
+See the discussion in https://github.com/pypa/pip/pull/9945.

src/pip/_internal/req/req_install.py

@@ -509,30 +509,16 @@ def load_pyproject_toml(self):
  self.unpacked_source_directory, backend, backend_path=backend_path,
  )
 
- def _check_setup_py_or_cfg_exists(self) -> bool:
- """Check if the requirement actually has a setuptools build file.
-
- If setup.py does not exist, we also check setup.cfg in the same
- directory and allow the directory if that exists.
- """
- if os.path.exists(self.setup_py_path):
- return True
- stem, ext = os.path.splitext(self.setup_py_path)
- if ext == ".py" and os.path.exists(f"{stem}.cfg"):
- return True
- return False
-
  def _generate_metadata(self):
  # type: () -> str
  """Invokes metadata generator functions, with the required arguments.
  """
  if not self.use_pep517:
  assert self.unpacked_source_directory
 
- if not self._check_setup_py_or_cfg_exists():
+ if not os.path.exists(self.setup_py_path):
  raise InstallationError(
- f'File "setup.py" or "setup.cfg" not found for legacy '
- f'project {self}.'
+ f'File "setup.py" not found for legacy project {self}.'
  )
 
  return generate_metadata_legacy(