ci: remove dead Python 3.8 condition from the mypy step

The `Check types with mypy` step in the `test` job of
`pythonpackage.yml` ran only `if: matrix.python-version != '3.8'`.
That condition was added in #2087, alongside a `python_version >= "3.9"`
marker on mypy in `test-requirements.txt`. Since mypy was not
installed on older Pythons, the step had to be skipped there. (The
same pull request also pinned mypy to 1.18.2, which itself requires
Python 3.9 or later.) The condition originally excluded 3.7 as well,
and that part was removed along with 3.7 support.

Since Python 3.8 through 3.10 were moved out of the `test` job's
matrix into the `install/import` job, which does not run mypy, the
`test` job runs only on 3.11 through 3.15t. No row of its matrix, and
no `include` entry, uses 3.8, so the condition is always true. It
suggested that the job still covers 3.8 and did nothing else.

Remove it. The step runs in exactly the same jobs as before.

The `python_version >= "3.9"` marker on the mypy requirement is left
as it is. It is packaging metadata for the `test` extra, not CI
configuration.

`actionlint` reports no problems, and zizmor 1.31.1 at the `pedantic`
level reports no new findings.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -207,7 +207,6 @@ jobs:
       continue-on-error: true
 
     - name: Check types with mypy
-      if: matrix.python-version != '3.8'
       run: |
         mypy --python-version="${PYTHON_VERSION%t}"  # Version only, with no "t" for free-threaded.
       env: