Numeric literal formatting is inconsistent with Black code style document

[Jackenmen]

According to Numeric literals section of Black code style, the numeric literals should use lowercase letters for the syntactic parts and uppercase letters for the digits themselves but this seems to no longer be the case on master (has been changed in #1775). I don't really agree with that change but nonetheless, the implementation should be consistent with the documentation so it looks like one of these needs changing.

I used a blank issue as this very well may just be a documentation issue.

[JelleZijlstra]

The current documentation in https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#numeric-literals is consistent with Black's actual behavior. (We changed back to uppercase for hex digits.)