Add matching for f-string interpolation brackets

[bugeats]

I find it difficult to see at a glance where f-string interpolation is happening.

Where:

print(f'hello {name}')

would add syntax matching for the { and } characters.

Also, even though general syntax highlighting for the interpolated name expression seems to be beyond vim's capabilities at the moment, it might be nice to match "everything inside the brackets" too.