test: Add pytest infrastructure and attachment tests

In making my last fix to attachments, I found it challenging not
having tests to ensure there was no regression.

Added pytest with minimal setup and isolated configuration. Created
a separate test workflow to keep tests isolated from linting.

Tests cover the key elements of the attachment logic:
- URL extraction from issue bodies
- Filename extraction from different URL types
- Filename collision resolution
- Manifest duplicate prevention

release-requirements.txt

@@ -8,6 +8,7 @@ colorama==0.4.6
 docutils==0.22.3
 flake8==7.3.0
 gitchangelog==3.0.4
+pytest==8.3.3
 idna==3.11
 importlib-metadata==8.7.0
 jaraco.classes==3.4.0