Combine f-strings which would be placed on the same line

[pushfoo]

Describe the style change

Combine short f-string segements which would be placed on the same line

Examples in the current Black style

1. Input: use a multi-part f-string shorter than line length

   sep= " "
   FSTRING = (
       f"many"
       f"{sep}"
       f"tiny"
       f"{sep}"
       f"fstrings"
   )

2. Run it locally or via this vercel example
3. The output wastes horizontal space: it puts them on one line, but not joined together

   sep= " "
   FSTRING = f"many" f"{sep}" f"tiny" f"{sep}" f"fstrings"

Desired style

sep = " "
FSTRING = f"many{sep}tiny{sep}fstrings"

Additional context
Originally found while reviewing a PR for Python Arcade, a game framework and educational tool.