Change
```python
"""Test docstring"""
a = 1
```
to
```python
"""Test docstring"""

a = 1
```
in preview style, but don't touch the docstring otherwise.

Do we want to ask black to also format the content of module level docstrings? Seems inconsistent to me that we change function and class docstring indentation/contents but not module docstrings.

Fixes #7995