[electrum] bump minimum required version of trezorlib to 0.13.8

Summary:
This seems to be enough to support the new Trezor Safe 3 devices, according to tests done by Electron Cash and Electrum users.
I did not test the new device myself.

See electrum#8722 and electroncash#2757

Test Plan:
Install that version of the trezor library for python and test that trezor wallets still work in the application (tested with a Trezor T).

pip install trezor==0.13.8
./electrum-abc

I also tested the appimage (contrib/build-linux/appimage/build.sh)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15197