Preferentially use certifi to requests.

requests/certs.py

@@ -11,14 +11,15 @@ If you are packaging Requests, e.g., for a Linux distribution or a managed
 environment, you can change the definition of where() to return a separately
 packaged CA bundle.
 """
-
 import os.path
 
-
-def where():
-    """Return the preferred certificate bundle."""
-    # vendored bundle inside Requests
-    return os.path.join(os.path.dirname(__file__), 'cacert.pem')
+try:
+    from certifi import where
+except ImportError:
+    def where():
+        """Return the preferred certificate bundle."""
+        # vendored bundle inside Requests
+        return os.path.join(os.path.dirname(__file__), 'cacert.pem')
 
 if __name__ == '__main__':
     print(where())