Do not run doctests on the wheels

To avoid errors like these when cibuildwheel runs pytest on the installed
wheel:

```
_____________ ERROR collecting src/cutadapt/cli.py _____________
import file mismatch:
imported module 'cutadapt.cli' has this __file__ attribute:
  .../cutadapt/.venv/lib/python3.12/site-packages/cutadapt/cli.py
which is not the same as the test file we want to collect:
  .../cutadapt/src/cutadapt/cli.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
```

.github/workflows/ci.yml

@@ -83,6 +83,7 @@ jobs:
       uses: pypa/[email protected]
       env:
         CIBW_BUILD: "cp*-manylinux_x86_64 cp3*-win_amd64 cp3*-macosx_x86_64 cp3*-macosx_arm64"
+        PYTEST_ADDOPTS: "--doctest-ignore-import-errors"
     - uses: actions/upload-artifact@v4
       with:
         name: wheels-${{ matrix.os }}