fix: Python 3.6 compatibility for tests

- Replace walrus operator (:=) with traditional assignment in config.py
- Replace datetime.fromisoformat() with strptime() for Python 3.6
- Replace subprocess capture_output and text params with PIPE and universal_newlines
- All tests now pass on Python 3.6.9

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tddai/issue_fetcher.py

@@ -4,6 +4,7 @@ Issue fetching from Gitea API.
 
 import json
 import subprocess
+from subprocess import PIPE
 from dataclasses import dataclass
 from datetime import datetime
 from typing import List, Optional, Dict, Any
@@ -42,8 +43,9 @@ class IssueFetcher:
         try:
             result = subprocess.run(
                 ['curl', '-s', f"{self.config.issues_api_url}/{issue_number}"],
-                capture_output=True,
-                text=True,
+                stdout=PIPE,
+                stderr=PIPE,
+                universal_newlines=True,
                 check=True
             )
 
@@ -71,8 +73,9 @@ class IssueFetcher:
 
             result = subprocess.run(
                 ['curl', '-s', url],
-                capture_output=True,
-                text=True,
+                stdout=PIPE,
+                stderr=PIPE,
+                universal_newlines=True,
                 check=True
             )
 
@@ -111,8 +114,8 @@ class IssueFetcher:
                 title=issue_data['title'],
                 body=issue_data.get('body', ''),
                 state=issue_data['state'],
-                created_at=datetime.fromisoformat(issue_data['created_at'].replace('Z', '+00:00')),
-                updated_at=datetime.fromisoformat(issue_data['updated_at'].replace('Z', '+00:00')),
+                created_at=datetime.strptime(issue_data['created_at'].replace('Z', '').split('.')[0], '%Y-%m-%dT%H:%M:%S'),
+                updated_at=datetime.strptime(issue_data['updated_at'].replace('Z', '').split('.')[0], '%Y-%m-%dT%H:%M:%S'),
                 html_url=issue_data['html_url'],
                 assignee=assignee,
                 labels=labels