📦 Move packaging to PEP 517 in-tree backend

This essentially allows the cythonization opt-out be controlled by the
`pure-python` PEP 517 config setting that can be passed to
the corresponding build frontends via their respective CLIs.

MANIFEST.in

@@ -6,6 +6,7 @@ include CHANGES.rst
 include README.rst
 include Makefile
 graft multidict
+graft packaging
 graft docs
 graft CHANGES
 graft requirements

packaging/README.md

@@ -0,0 +1,11 @@
+# `pep517_backend` in-tree build backend
+
+The `pep517_backend.hooks` importable exposes callables declared by PEP 517
+and PEP 660 and is integrated into `pyproject.toml`'s
+`[build-system].build-backend` through `[build-system].backend-path`.
+
+# Design considerations
+
+`__init__.py` is to remain empty, leaving `hooks.py` the only entrypoint
+exposing the callables. The logic is contained in private modules. This is
+to prevent import-time side effects.

packaging/pep517_backend/__init__.py

@@ -0,0 +1 @@
+"""PEP 517 build backend for optionally pre-building Cython."""

packaging/pep517_backend/__main__.py

@@ -0,0 +1,6 @@
+import sys
+
+from . import cli
+
+if __name__ == "__main__":
+    sys.exit(cli.run_main_program(argv=sys.argv))