chore: modernize SDK to Python 3.8+ baseline

This change aligns the library with modern Python standards by dropping
support for legacy Python versions (2.7 and 3.6).

Technical Changes:
- Updated setup.py to set python_requires='>=3.8' and added classifiers
  up to Python 3.14.
- Introduced PEP 484 Type Hints in AdyenClient and AdyenResult to
  improve developer experience and IDE support.
- Removed legacy __future__ imports and (object) inheritance.
- Modernized super() calls to use Python 3 syntax.
- Updated setup.py with install_requires and extras_require for
  idiomatic dependency management (requests, pycurl).
- Updated tox.ini and GitHub Actions to test across Python 3.8 through 3.14.