Fix codespell and blacken-docs failures in CHANGELOG.md

[codechrl]

pre-commit.ci runs with --all-files, so these two failures currently show up on every open pull request, not just one that touches CHANGELOG.md. As of now they red #2290, #2294, #2299 and #2301.

Two fixes, both in CHANGELOG.md:

• codespell — configureable → configurable (line 18). That entry is generated from the title of Allow configureable WebPDF JavaScript processing timeout #2250, so the typo can only be corrected here.
• blacken-docs — a Python block in the 5.6.1 section was missing the blank line black requires before a top-level statement.

Verification

Run against the versions pinned in .pre-commit-config.yaml, not whatever was newest:

hook	pinned version	before	after
codespell	2.4.2, -L sur,nd,assertin	exit 65	exit 0
blacken-docs	1.20.0 + black==23.7.0	exit 1	exit 0
mdformat	1.0.0	clean	clean, file unchanged

Also checked that the edited lines carry no trailing whitespace and the file still ends in a single newline, so trailing-whitespace and end-of-file-fixer stay happy.

No code or behaviour changes.

[codechrl]

CI is 27/28 green. The only red is enforce-label, which needs a triage label — maintenance looks right. I can't apply it from a fork (403, needs write access), so this needs a maintainer.

Worth noting the blast radius: pre-commit.ci runs with --all-files, so these two CHANGELOG.md findings currently fail on every open PR, not just one that touches the file. #2290, #2294, #2299 and #2301 are all red for this reason rather than anything their authors did.

(The run-tests (macos-latest, 3.13) failure on the first run was test_qtpng.py::test_export returning b'' — already marked @pytest.mark.flaky, and flaky had retried it twice. It passed on re-run, and this PR changes no code.)

[codechrl]

@minrk sorry for the direct ping — this just needs a maintenance label to clear enforce-label (everything else is green). Flagging it because it unblocks four PRs at once, not just this one.