Add type hints for firewall.py

[hippalectryon-0]

Follows #440

[codecov]

Codecov Report

❌ Patch coverage is 99.01961% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.36%. Comparing base (71fab5b) to head (6199cf1).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
qubesadmin/devices.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #441      +/-   ##
==========================================
+ Coverage   76.35%   76.36%   +0.01%     
==========================================
  Files          53       53              
  Lines        9388     9398      +10     
==========================================
+ Hits         7168     7177       +9     
- Misses       2220     2221       +1     

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