update black

[ThomasWaldmann]

we use an older black tool, see requirements.d/*.txt.

updating it needs:

• a time with few big PRs open (avoiding causing conflicts)
• a PR updating the black version requirement in one commit and also updating the complete borg src in another commit

[ThomasWaldmann]

Note: Updating to black 26.3 creates a >1500 lines long git diff.

[ThomasWaldmann]

Alternative idea: use ruff.