Fix Python version requirement to 3.10+

Updated minimum Python version from 3.9 to 3.10 to match the badge and actual requirements.

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

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

Author: ty13r

README.md

@@ -11,7 +11,7 @@
 
 ## 📋 Requirements
 
-- Python 3.9 or higher
+- Python 3.10 or higher
 - Substack account credentials:
     - Email and password (recommended)
     - OR session token and user ID