Make Result.serialize work more like Graph.serialize

[aucampia]

DO NOT REVIEW: I'm going to split everything off from this that I can to reduce it to just the breaking changes so we can merge it before 7.0.0.

Reviewers beware: This patch is pretty big and contains a lot of things that could be merged separately. I may at some point break off some of the changes to separate PRs.

This patch makes the following changes to Result.serialize.

• Return str by default instead of bytes.
• Use "txt" as the default tabular serialization format.
• Use "turtle" as the default graph serialization format.
• Support both typing.IO[bytes] and typing.TextIO destinations.

Corresponding changes are made to the specific serializers also.

This patch also changes how text is written to typing.IO[bytes] in
serializers to ensure that the buffer is flushed and
detatched from the TextIOWrapper once the serialization function
completes so it can be used normally afterwards.

This patch further includes a bunch of additional type hints.

Fixes #1338

[aucampia]

@nicholascar as this breaks the interface I will try to break of a bunch of these changes and merge them independently so that this can be minimal in size and deferred to 7.x.

[nicholascar]

@aucampia since you are doing work elsewhere, do you want to put this PR on hold or to close it perhaps?

[aucampia]

@nicholascar it's somewhat up to you. I feel this is ready to merge, if you only want to merge things that should go into 6.x then it is probably best to defer this, in which case I will proceed to break parts of this off into other PRs that won't break the interface and then proceed to have them merged. The other option is maybe to make a 6.x branch and then have master for 7.x but this may be more effort than it is worth.

If we keep master for 6.x until we are ready to start work on 7.x then I will mark this as draft and proceed with merging the parts of this that does not break the interface and rebase this appropriately so that it has a minimal diff.

[aucampia]

I will anyway rebase this as other PRs are merged and keep this in a mergeable state at all times as best I can.

[nicholascar]

@aucampia I see your other PRs with some of these parts in them, e.g. the type hinting additions. If we merge all those, I guess that will show the interface-breaking part here better. Then we can decide to merge into 6.0.x or 6.x.x or 7.x.

So let's just wait on this for all those type hinting ones to go through!

[aucampia]

Closing PR, no need to keep it open, I'm busy splitting stuff off though but it just clogs up the view.