My own test broke CI, and the way it broke is worth more than the fix.

It imported `tomllib` at module scope. That is stdlib only from 3.11, and this
package supports 3.10 -- requires-python = ">=3.10", and the wire-format matrix
tests it. So on the 3.10 leg the import raised, and pytest reported:

    collecting ... collected 69 items / 1 error
    ERROR tests/test_version_is_single_sourced.py
    E   ModuleNotFoundError: No module named 'tomllib'
    !!!!!!!! Interrupted: 1 error during collection !!!!!!!!

A collection error is not one failing test -- it aborts the ENTIRE session. All
69 wire-format tests stopped running on 3.10, and the job surfaced as a single
red check, indistinguishable from one ordinary failure. A green-looking matrix
with one red leg hid "68 tests never ran".

Fix: import the parser inside the function that uses it, with a 3.10 branch to
the `tomli` backport. Local scope means a parser problem fails THIS test and
leaves the rest of the suite running.

Verified against a real CPython 3.10.20, not assumed:
  - before: Interrupted, 0 tests run
  - after:  53 passed, 19 skipped -- and all three version tests RUN, not skip
  - mutation (break the in-function import deliberately): 2 failed, 51 passed,
    no Interrupt -- which is the property the change is actually for
  - 3.12 unchanged: 53 passed, 19 skipped

Also declares `tomli>=2 ; python_version < "3.11"` in the dev extra. Stated
precisely: this is NOT what fixes the failure. pytest already depends on tomli
on <3.11 (it parses pyproject.toml config with it), so it was installed the
whole time -- removing it stops pytest from starting at all, before collection.
The declaration is so the suite names its own dependency instead of resting on
a transitive dependency of the test runner.

build(deps-dev): update @typescript-eslint/eslint-plugin requirement from ^8.66.0 to ^8.67.0