tox.ini: Cleanup obsolete py27 test environments

[bernhardkaindl]

Summary

tox.ini: Cleanup obsolete test environments (from combining Python 2.7 coverage with 3.x coverage)

While at it, also add min_version = 4.6 (4.5 had a bug that needed workarounds previously)

Pull Request Overview by GitHub Copilot Review

This PR removes obsolete Python 2.7 test environments and configuration from the tox setup. The changes update the minimum tox version requirement and clean up all Python 2.7-related dependencies, test environments, and documentation references.

• Removes all Python 2.7-specific test environments and dependencies
• Updates minimum tox version to 4.6 and simplifies development setup
• Cleans up obsolete coverage combination functionality that was specific to Python 2.7

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request.

File	Description
tox.ini	Removes py27 environments, covcombine section, and updates configuration for modern tox
pyproject.toml	Adds test dependencies to coverage, mypy, and pytype extras
README.md	Removes Python 2.7 support references and simplifies CI description
CONTRIBUTING.md	Removes Python 2.7 development setup instructions and simplifies tox installation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #167   +/-   ##
======================================
  Coverage    83.3%   83.3%           
======================================
  Files          23      23           
  Lines        3347    3347           
======================================
  Hits         2790    2790           
  Misses        557     557           

[coveralls]

Pull Request Test Coverage Report for Build 17270478682

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.358%

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Totals
Change from base Build 17124120104:	0.0%
Covered Lines:	2790
Relevant Lines:	3347

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💛 - Coveralls