gh-67041: Allow to distinguish between empty and not defined URI components

[serhiy-storchaka]

Changes in the urllib.parse module:

• Add option allow_none in urlparse(), urlsplit() and urldefrag(). If it is true, represent not defined components as None instead of an empty string.
• Add option keep_empty in urlunparse() and urlunsplit(). If it is true, keep empty non-None components in the resulting string. By default it is the same as the allow_none value for the result of the urlparse() and urlsplit() calls.
• Add option keep_empty in the geturl() method of DefragResult, SplitResult, ParseResult and the corresponding bytes counterparts.

• Issue: urllib.parse wrongly strips empty #fragment, ?query, //netloc #67041

[serhiy-storchaka]

It is now ready to review. The status of allow_none is now saved in the DefragResult, SplitResult and ParseResult objects, so in most cases there is no need to pass the keep_empty argument. geturl() no longer needs the keep_empty parameter.

Unfortunately, these objects now have __dict__ and no longer immutable. This is because non-empty __slots__ is not compatible with tuple subclasses. This is a separate complex issue. I'll try to find a solution of it, but it may be difficult.

The long term plan is to make allow_none True by default, and later deprecate allow_none=False. keep_empty=False can still be useful.

[serhiy-storchaka]

I am sorry, I forget to copy the _keep_empty attribute in copying/encoding/decoding methods. Now the PR is ready for review.

@orsenthil, @barneygale, could you please make a review?