Update arxiv.py

arxiv.py

@@ -13,6 +13,9 @@ def get_paper(paper_url):
 
     suffix = 'paper-dir/' + eprint_url.replace("https://arxiv.org/e-print/", "")
 
+    if not os.path.exists("paper-dir"):
+        call(["mkdir", 'paper-dir'])
+
     # check if the directory exists
     if os.path.exists(suffix):
         print("Paper already downloaded, skipping download")