Drop Python 2.7, 3.7, 3.8 support

[Copilot]

This PR removes support for Python 2.7, 3.7, and 3.8 as requested in the issue. These Python versions are outdated and no longer supported upstream.

Changes made:

Configuration updates:

• setup.py: Updated python_requires from >=3.7 to >=3.9 and removed Python 3.7, 3.8 classifiers
• .github/workflows/test.yaml: Removed Python 3.7 and 3.8 from the CI test matrix
• doc/index.rst: Updated documentation to reflect "Python 3.9+" instead of "Python 2.7, 3.4+"

Code cleanup:

• tests.py: Removed Python 2 compatibility code including:
  • from __future__ import unicode_literals import
  • Python 2/3 version checks in test_str_and_repr method
  • Unused sys import
• .coveragerc: Updated comment from "Python 2/3 compatibility" to "Import fallbacks"

All tests continue to pass and the codebase is now cleaner without the legacy compatibility code.

Fixes #48.

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