B2 SDK contributor docs describe a stale Python test matrix #589

Description

The contributor guides state that CI tests a matrix of eight Python versions. The current configuration names CPython 3.10 through 3.14 and PyPy 3.10, for six interpreter versions across supported operating systems.

Steps to reproduce

1. Read the CI-matrix claim in CONTRIBUTING.md or doc/source/contributing.rst.
2. Compare it with PYTHON_VERSIONS in noxfile.py and the matrix in .github/workflows/ci.yml.

Expected behavior

Contributor documentation should describe the current interpreter and operating-system matrix accurately without hard-coded counts that drift easily.

Actual behavior

The documented count does not match the current configuration.

Environment

• Python version: CPython 3.10-3.14 and PyPy 3.10
• b2sdk version: master at f3ab7b71
• OS: Linux, macOS, and Windows