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

Author: Iamrodos

.github/workflows/test.yml

@@ -0,0 +1,33 @@
+---
+name: "test"
+
+# yamllint disable-line rule:truthy
+on:
+  pull_request:
+    branches:
+      - "*"
+  push:
+    branches:
+      - "main"
+      - "master"
+
+jobs:
+  test:
+    name: test
+    runs-on: ubuntu-24.04
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+      - name: Setup Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: "pip"
+      - run: pip install -r release-requirements.txt
+      - run: pytest tests/ -v

pytest.ini

@@ -0,0 +1,6 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+addopts = -v

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

tests/__init__.py

@@ -0,0 +1 @@
+"""Tests for python-github-backup."""