core: replaced sat shell script by a python script:

the backend is now launched by `sat.core.launcher`, and the script is generated during
installation with a new entry point in `setup.py`.

setup.py

@@ -110,7 +110,12 @@ setup(
                  ["CHANGELOG", "COPYING", "INSTALL", "README", "README4TRANSLATORS"]),
                 (os.path.join("share", DBUS_DIR), [DBUS_FILE]),
                 ],
-    scripts=["sat_frontends/jp/jp", "sat_frontends/primitivus/primitivus", "bin/sat"],
+    entry_points={
+        "console_scripts": [
+            "sat = sat.core.launcher:Launcher.run",
+            ],
+        },
+    scripts=["sat_frontends/jp/jp", "sat_frontends/primitivus/primitivus"],
     zip_safe=False,
     setup_requires=["setuptools_scm"] if is_dev_version else [],
     use_scm_version=sat_dev_version if is_dev_version else False,