Have `setup.py clean` remove compiled Cython files

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/907

Differential Revision: D18480215

Pulled By: myleott

fbshipit-source-id: b02002f631f6d47380f309d4f464bd135d623280

setup.py

@@ -97,6 +97,13 @@ else:
     dependency_links = []
 
 
+if 'clean' in sys.argv[1:]:
+    # Source: https://bit.ly/2NLVsgE
+    print("deleting Cython files...")
+    import subprocess
+    subprocess.run(['rm -f fairseq/*.so fairseq/**/*.so'], shell=True)
+
+
 setup(
     name='fairseq',
     version='0.8.0',