Make transformers-cli cross-platform (#4131)

* make transformers-cli cross-platform

Using "scripts" is a useful option in setup.py particularly when you want to get access to non-python scripts. However, in this case we want to have an entry point into some of our own Python scripts. To do this in a concise, cross-platfom way, we can use entry_points.console_scripts. This change is necessary to provide the CLI on different platforms, which "scripts" does not ensure. Usage remains the same, but the "transformers-cli" script has to be moved (be part of the library) and renamed (underscore + extension)

* make style & quality

setup.py

@@ -127,7 +127,9 @@ setup(
         "sacremoses",
     ],
     extras_require=extras,
-    scripts=["transformers-cli"],
+    entry_points={
+        "console_scripts": ["transformers-cli=transformers.commands.transformers_cli:main"]
+    },
     python_requires=">=3.6.0",
     classifiers=[
         "Development Status :: 5 - Production/Stable",