Fix fstring

[jarrodmillman]

Fix #6730

Before

$ python
Python 3.9.9 (main, Nov 19 2021, 00:00:00)
[GCC 10.3.1 20210422 (Red Hat 10.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> remaining = ["t", "s"]
>>> msg = f"No warning raised matching:\n{{'\n'.join(remaining)}}"
>>> print(msg)
No warning raised matching:
{'
'.join(remaining)}

After

$ python
Python 3.9.9 (main, Nov 19 2021, 00:00:00)
[GCC 10.3.1 20210422 (Red Hat 10.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> remaining = ["t", "s"]
>>> newline = "\n"
>>> msg = f"No warning raised matching:{newline}{newline.join(remaining)}"
>>> print(msg)
No warning raised matching:
t
s