Fix typing for `setdefault`

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.58%. Comparing base (c92f8a6) to head (01b8b88). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/pytest_bdd/utils.py	25.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #730 +/- ## ========================================== - Coverage 95.63% 95.58% -0.05% ========================================== Files 54 54 Lines 2083 2084 +1 Branches 231 230 -1 ========================================== Hits 1992 1992 - Misses 55 56 +1 Partials 36 36 ```

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