Address `ruff` formatting issues for local development

Why are we doing this?

When on the dependabot rota, I reviewed a PR for a version bump for Ruff from 0.16.3 to 0.16.4.

This was a patch release that passed our CI checks, was merged, and deployed successfully. However, after pulling down the branch and running just fix locally (which runs the Ruff linter), I found that Ruff reported two remaining errors:

$ just fix
/home/katie/bin/just _compile requirements.prod.in requirements.prod.txt 
/home/katie/bin/just _compile requirements.dev.in requirements.dev.txt 
$BIN/ruff check --fix .
RUF015 Prefer `next(iter(section.keys()))` over single element slice
  --> hooks/ehrql_branch.py:22:12
   |
20 |         (i, section)
21 |         for i, section in enumerate(config["nav"])
22 |         if list(section.keys())[0] == "ehrQL"
   |            ^^^^^^^^^^^^^^^^^^^^^^^
23 |     )
24 |     new_import_string = ehrql_nav_section["ehrQL"].replace(
   |
help: Replace with `next(iter(section.keys()))`

INP001 File `scripts/wordcount.py` is part of an implicit namespace package. Add an `__init__.py`.
--> scripts/wordcount.py:1:1

Found 4 errors (2 fixed, 2 remaining).
No fixes available (1 hidden fix can be enabled with the `--unsafe-fixes` option).
error: recipe `fix` failed on line 123 with exit code 1

What are we doing

Resolve the Ruff lint failures introduced by the ruff upgrade.

How will we know when it's done?

just fix completes successfully with no remaining Ruff errors, and the existing CI checks continue to pass.

Start with the Ruff output and inspect hooks/ehrql_branch.py:22 and scripts/wordcount.py:1, the two files named in the failing diagnostics. Run just fix before and after the changes, then confirm it completes with no Ruff errors and that the existing CI checks still pass.