[electrum] support trezor==0.13.9

Summary:
The 0.13.9 version of trezor released last week breaks the Trezor plugin.
This does not affect recent releases, which were packaged with 0.13.8 (see contrib/deterministic-build/requirements-hw.txt)

For now it is best to keep supporting 0.13.8 and keep using it for releases because I only did basic tests with 0.13.9. I will keep testing the new version of the library for a while before dropping support for 0.13.8.

Test Plan:

pip install trezor==0.13.8
python test_runner.py
pip install trezor==0.13.9
python test_runner.py

Open a wallet with a Trezor device, connect to the device, sign a transaction.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16406