Deprecating Python 3.7 for click 9.0

[saroad2]

Python 3.7 is reaching its end-of-life date.

This issue is here in order to make sure that we deprecate the use of Python 3.7 for the next major release of click.
This will allow us to use new neat features introduced in Python 3.8:

• The walrus operator
• positional-only and keywords-only parameters
• f-strings with equal signs

[davidism]

This will be done with 8.2, but I don't typically keep issues open to track it.