[electrum] minor cleanup of Transaction.fetch_input_data

Summary:

• put comments on previous line to let the automatic code formatting do its job
• save some indentation levels
• add a few typehints
• use f-strings
• tx_cache_put is always called with a valid Transaction, no need to check for tx is not None
• move imports to the top of the file (they are always imported anyway by other modules, and multiple imports add no significant additional cost)

Test Plan: send a transaction between two different wallets; in the receiving wallet double-click on the tx in the history, check that the dialog knows the input amount

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14514