Fix signify usage

[kmathisbf]

Overnight, upstream changes in signify broke usage of the SignedPEFile class. These changes support the newest version of v0.9.1.

Fixes error:

19.36 Traceback (most recent call last):
19.36   File "<string>", line 1, in <module>
19.36   File "/tmp/EMBER2024/src/thrember/__init__.py", line 1, in <module>
19.36     from .features import (
19.36   File "/tmp/EMBER2024/src/thrember/features.py", line 28, in <module>
19.36     from signify.authenticode import SignedPEFile
19.36 ImportError: cannot import name 'SignedPEFile' from 'signify.authenticode' (/var/lang/lib/python3.12/site-packages/signify/authenticode/__init__.py). Did you mean: 'signed_file'?