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
```

pyproject.toml

@@ -63,7 +63,7 @@ write_to = "src/cutadapt/_version.py"
 warn_unused_configs = true
 
 [tool.cibuildwheel]
-environment = "CFLAGS=-g0"
+environment = {"CFLAGS" = "-g0", "PYTEST_ADDOPTS" = "--doctest-ignore-import-errors"}
 test-extras = ["dev"]
 test-command = ["pytest {project}"]