[electrum] fix OutPoint.str

Summary:
I added an extra parenthesis when implementing this function in D14432, I started breaking things in a limited way D14472 by using str(txin.outpoint) (reading tx from file or from a pasted string), and it started getting really bad in D14520 when additional usage of str(txin.outpoint) in critical wallet code corrupted my test wallet file.

String outpoints are used in the wallet to locate inputs.

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14521