Breaking Change: Removed obsolete/duplicate setup.py from Python.

This copy of `setup.py` is obsolete, as it builds the old C++ backend for Protobuf, which has been deprecated and
unused since 4.21.0 when
[the backend was switch to upb](https://protobuf.dev/news/2022-05-06/#python-updates).

The `setup.py` that we actually distribute in our source packages is located in [`python/dist/setup.py`](https://github.com/protocolbuffers/protobuf/blob/main/python/dist/setup.py).  It is not possible to build this `setup.py` directly from the GitHub repo or GitHub release tarball, because it depends on the file layout of our Python source package ([as distributed on PyPI](https://pypi.org/project/protobuf/#files)).  The Python source package uses a layout that pulls together all of the things Python needs:

|Python Source Package Path|GitHub Repo Path|Description|
|-----|-----|-----|
|`setup.py`|`python/dist/setup.py`|
|`google/protobuf/*`|`python/google/protobuf/*`|pure Python sources|
|`python/*`|`python/*`|C extension sources|
|`utf8_range/*`|`third_party/utf8_range`|C UTF-8 Validation Library|
|`upb/*`|`upb/upb/*`|C Protobuf Library|

Users who want to build their own Python packages should build from our source package on PyPI, not from our GitHub repo or our GitHub release tarball.

It is also possible to build our source package from GitHub using the following command (this requires Bazel):

```
$ bazel build //python/dist:source_wheel
```

PiperOrigin-RevId: 603118807

python/README.md

@@ -2,6 +2,11 @@
 
 This directory contains the Protobuf library for Python.
 
+For user documentation about how to use Protobuf Python, see
+https://protobuf.dev/getting-started/pythontutorial/
+
+# Installation
+
 In most cases you should install the library using `pip` or another package
 manager:
 
@@ -12,10 +17,7 @@ $ pip install protobuf
 The packages released on https://pypi.org/project/protobuf/#files include both a
 source distribution and binary wheels.
 
-For user documentation about how to use Protobuf Python, see
-https://protobuf.dev/getting-started/pythontutorial/
-
-# Building packages from this repo
+## Building packages from this repo
 
 If for some reason you wish to build the packages directly from this repo, you
 can use the following Bazel commands:
@@ -29,6 +31,15 @@ The binary wheel will build against whatever version of Python is installed on
 your system. The source package is always the same and does not depend on a
 local version of Python.
 
+## Building from `setup.py`
+
+We support building from `setup.py`, but only from a Python source package.
+You cannot build from `setup.py` using the GitHub repo or the GitHub source
+tarball.
+
+To build a source package from this repo, see the instructions in the previous
+section.
+
 # Implementation backends
 
 There are three separate implementations of Python Protobuf. All of them offer

python/build_targets.bzl

@@ -461,7 +461,6 @@ def build_targets(name):
             ":python_src_files",
             "README.md",
             "google/__init__.py",
-            "setup.cfg",
         ],
         strip_prefix = "",
         visibility = ["//python/dist:__pkg__"],
@@ -485,8 +484,6 @@ def build_targets(name):
             "google/protobuf/python_protobuf.h",
             "internal.bzl",
             "python_version_test.py",
-            "setup.cfg",
-            "setup.py",
         ],
         strip_prefix = strip_prefix.from_root(""),
         visibility = ["//pkg:__pkg__"],