black does not format expressions inside f-string

[werenike]

Describe the bug
Black does not seem to format expressions inside f-strings.

To Reproduce
Example file contents:

1+2
f"{1+2}"

Result after running black:

1 + 2
f"{1+2}"

Expected behavior
Expected result after formatting:

1 + 2
f"{1 + 2}"

Environment:

• Version: 19.10b0
• OS and Python version: Arch Linux, Python 3.8.0

Does this bug also happen on master?
Yes (tried in online version)

[JelleZijlstra]

Duplicate of #567 (although this issue actually has a better title).