Fix handling of SMS OTP whose first digit is 0 (#87)

The cast to int was removing that digit, but it's meaningful.

Without this change, it just keeps requesting the same OTP without
sending a new one, and never accepting the user input.

Author: dequis

n26/api.py

@@ -452,7 +452,9 @@ def _complete_authentication_flow(self, mfa_token: str) -> dict:
             mfa_response_data['grant_type'] = "mfa_otp"
 
             hint = click.style("Enter the 6 digit SMS OTP code", fg="yellow")
-            mfa_response_data['otp'] = click.prompt(hint, type=int)
+
+            # type=str because it can have significant leading zeros
+            mfa_response_data['otp'] = click.prompt(hint, type=str)
         else:
             mfa_response_data['grant_type'] = "mfa_oob"