[electrum] access TxOutput value by attribute name rather than index in coinchooser

Summary: TxOutput is a NamedTuple, which can be accessed by index or by attribute name. Accessing the fields by attribute name makes the code more readable.

Test Plan:
python test_runner.py

Run the application, send a transaction to make sure the codepaths are triggered.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14420