[docs] Fix documentation inconsistencies: Python 3.9 minimum and pyproject.toml references

[github-actions]

Summary

This PR fixes a set of documentation inconsistencies found across the doc/ folder by verifying each claim against the current source code and repository structure.

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Changes

doc/dev/tests.md

• Python version: Updated minimum version from 3.8 → 3.9, consistent with azure-core (requires-python = ">=3.9"), the sdk/template package (requires-python = ">=3.9"), and tool_usage_guide.md ("The monorepo requires a minimum of python 3.9").
• Mandatory files list: Replaced setup.py and setup.cfg (no longer present in the template package) with pyproject.toml, which is what the sdk/template/azure-template package uses.
• venv example: Updated the example path from Python38 to Python39.

doc/dev/packaging.md

• Updated "Wheels must work with Python 3.8+" constraint to 3.9+.
• Updated python_requires=">=3.8" to >=3.9 in the example setup.py snippet (two occurrences).
• Removed the Python 3.8 classifier from the example setup.py classifiers list.
• Updated the "updating to Python 3 only" section to say >=3.9.

doc/dev/static_type_checking.md

• py.typed configuration: Replaced the setup.py-based instruction (include_package_data=True + package_data) with the pyproject.toml-based equivalent ([tool.setuptools.package-data] with pytyped = ["py.typed"]), linking to the template pyproject.toml as a reference. Packages like azure-core and azure-template no longer ship a setup.py.
• typing-extensions example link: Replaced the stale historical commit SHA link pointing to setup.py#L73 with a link to the current pyproject.toml for azure-eventhub on main. Also removed the outdated mention of "Python 3.7+" backport support (minimum is now 3.9).

doc/eng_sys_checks.md

• Removed "right next to each package's setup.py" from the pyproject.toml section description — packages now use pyproject.toml only, not alongside setup.py.
• Updated depends, latestdependency, and mindependency check descriptions: replaced "setup.py" with "pyproject.toml" as the source of package requirements.

doc/tool_usage_guide.md

• Updated verifysdist description from "correct within the setup.py" to "correct within the package configuration".

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Verification

Each change was verified against:

• sdk/template/azure-template/pyproject.toml — uses requires-python = ">=3.9", has no setup.py
• sdk/core/azure-core/pyproject.toml — uses requires-python = ">=3.9", [tool.setuptools.package-data] for py.typed
• doc/tool_usage_guide.md — already states "The monorepo requires a minimum of python 3.9"
• sdk/eventhub/azure-eventhub/pyproject.toml — contains the typing-extensions>=4.0.1 dependency

Generated by Documentation Consistency Check · ◷

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