List of backward incompatible changes that could be made if proposed to packaging

[henryiii]

I'd like to track the changes that could be made if this is proposed to packaging (#103).

• Validate one-line entries for things like the SUMMARY (description) Validate that description is single line #30
• Use the Email module's EmailMessage Use email.message.Message instead of rolling own (RFC822Message) #101 (Edit: this might be tricky, but maybe we could clearly state bytes()/.as_bytes() is broken, or we could give up and do the same trick as here)
• Error on unknown fields by default Make it possible to know about "unknown" keys that might be set in [project] table #12
• Require the metadata version to be specified Always generated Metadata 2.2 #89
• Could use ExceptionGroups by default (will be opt-in for us) feat: all_errors #170

I'd also like to normalize the strings to use f"{value!r}" instead of f'"{value}"', which works better when a value is an int, for example, and more closely matches Python's output. This probably can be done in the main lib, though.

I think the packaging version could use packaging.metadata, and we could instead extend that with writing support?