[electrum] No python 3.12 syntax

Summary: The delete parameter of the TemporaryDirectory constructor has been introduced in python 3.12. Let's not use that.

Test Plan: Check updating to latest Trezor firmware still works.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17824