chore(deps): upgrade mypy and pytest with Python 3.9 version markers

[jericht]

Summary

Upgrade mypy (to 2.1.), pytest (to 9.1.), and black (to 26.*) per dependabot PRs #322, #320, and #323, while maintaining Python 3.9 compatibility using environment markers.

Changes

requirements-testing.txt:

• mypy == 1.19.*; python_version == "3.9" / mypy == 2.1.*; python_version >= "3.10"
• pytest == 8.4.*; python_version == "3.9" / pytest == 9.1.*; python_version >= "3.10"
• black == 25.*; python_version == "3.9" / black == 26.*; python_version >= "3.10"

pyproject.toml:

• Remove python_version = 3.9 from [tool.mypy] — mypy now infers the target from the running Python. Required because mypy 2.x dropped support for targeting 3.9.

test/openjd/sessions_v0/test_session.py:

• Add assert session._logger.extra is not None to satisfy mypy 2.x stricter nullable type checking.

Context

mypy 2.x, pytest 9.x, and black 26.x all dropped Python 3.9 support. Rather than blocking the upgrades entirely, this uses pip environment markers to install the last compatible version on Python 3.9 and the latest on 3.10+.

Supersedes #320, #322, and #323.

[github-actions]

Removing python_version = 3.9 means mypy now targets whatever interpreter it runs under (Python 3.10+ per the new mypy == 2.1.* split) instead of the minimum supported version. As a result type-checking will no longer catch code that uses 3.10+-only syntax/APIs but must still run on 3.9 (which is still in requires-python = ">=3.9" and the classifiers). Consider keeping the target pinned to the floor, e.g. python_version = "3.9" (quoted, since mypy 2.x is stricter about the value type), so 3.9 incompatibilities are still flagged.