Use canonical extension for docs.

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+"Eggsecutable" Scripts
+----------------------
+
+.. deprecated:: 45.3.0
+
+Occasionally, there are situations where it's desirable to make an ``.egg``
+file directly executable.  You can do this by including an entry point such
+as the following::
+
+    setup(
+        # other arguments here...
+        entry_points={
+            "setuptools.installation": [
+                "eggsecutable = my_package.some_module:main_func",
+            ]
+        }
+    )
+
+Any eggs built from the above setup script will include a short executable
+prelude that imports and calls ``main_func()`` from ``my_package.some_module``.
+The prelude can be run on Unix-like platforms (including Mac and Linux) by
+invoking the egg with ``/bin/sh``, or by enabling execute permissions on the
+``.egg`` file.  For the executable prelude to run, the appropriate version of
+Python must be available via the ``PATH`` environment variable, under its
+"long" name.  That is, if the egg is built for Python 2.3, there must be a
+``python2.3`` executable present in a directory on ``PATH``.
+
+IMPORTANT NOTE: Eggs with an "eggsecutable" header cannot be renamed, or
+invoked via symlinks.  They *must* be invoked using their original filename, in
+order to ensure that, once running, ``pkg_resources`` will know what project
+and version is in use.  The header script will check this and exit with an
+error if the ``.egg`` file has been renamed or is invoked via a symlink that
+changes its base name.
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