Remove Python 3.7 from Black

[vcidst]

Proposed changes:

• Remove Python 3.7 formatting from Black

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[tmbo]

lgtm - did you run the formatting? i guess black might format some things differently

[vcidst]

@tmbo Yes, it didn't change anything

❯ make formatter
poetry run black rasa tests
('Configuration file exists at /Users/zi/Library/Application Support/pypoetry, reusing this directory.\n\nConsider moving TOML configuration files to /Users/zi/Library/Application Support/pypoetry, as support for the legacy directory will be removed in an upcoming release.',)
All done! ✨ 🍰 ✨
557 files left unchanged.