Modernize Validate_Email: Python 3+ support, multithreading, CSV bulk, smarter heuristics

[Reabot6]

This PR completely modernizes the Validate_Email package for 2025:

• Rewrote core validation logic for Python 3, removing legacy Python 2 code.
• Replaced pyDNS with dnspython for MX record checks and added requests for website availability.
• Added multithreading for CSV/bulk email validation for faster processing.
• Improved SMTP handling and heuristics for Gmail, Outlook, and general domains.
• CSV pipeline now produces pass/fail files with detailed validation reasons.
• Cleaned up caches and internal functions for maintainability.
• Preserved original MIT license and attribution to Syrus Akbary; all enhancements are clearly noted.

This update makes the package compatible with modern systems and large-scale email verification workflows.