Exceptions to comment spacing

[JelleZijlstra]

Black enforces that every comment starts with a space, except if the comment starts with a hardcoded list of black.comments.COMMENT_EXCEPTIONS (COMMENT_EXCEPTIONS = " !:#'"). These were added over time to support particular tools that treat certain comments specially. We occasionally get requests to treat more comments specially (#3557) or confusion because of the inclusion of particular characters (#3036). I am opening this issue to focus the discussion and arrive at an overall solution.

Options I'm considering:

1. Add additional exceptions as people ask for them. I don't want to do this; there is a potentially endless list of tools that treat certain comments specially, and the inconsistent behavior where certain characters suppress the leading space will continue to confuse users who don't have a reason to care about leading spaces in their comments.
2. Remove all the exceptions. Tell the third-party tools that rely on comments without leading spaces to change their behavior. This simplifies our life, but it's not very collegial.
3. Don't insert spaces before all non-alphanumeric characters (as suggested in Inconsistent adding of space after # in inline comments depending on single / double quotes used #3036 (comment)). This would make it so we don't have to worry about support for individual tools, but I'd worry that this makes Black's style less consistent: probably many comments in real-world code that start with punctuation without a leading space aren't intended for tools.
4. Bite the bullet and add a configuration option that allows users to control the COMMENT_EXCEPTIONS. We're generally hesitant to do this because it makes the Black code style less consistent, but I think in this case there's a reasonably strong argument: different projects use different tools, so it makes sense to have a different configuration for this option.