Convert Nonetype arguments to string before calling HTTPBasicAuth

This is for forward compatibility with requests 3.0.0
(see Line 28 in 2e51624)

Author: moseshassan

src/pip/_internal/download.py

@@ -332,7 +332,7 @@ def __call__(self, req):
 
         if username is not None or password is not None:
             # Send the basic auth with this request
-            req = HTTPBasicAuth(username, password)(req)
+            req = HTTPBasicAuth(username or "", password or "")(req) # Avoid passing Nonetype
 
         # Attach a hook to handle 401 responses
         req.register_hook("response", self.handle_401)