Add support for dt.timedelta to travel class

[AgDude]

Adds support to pass a datetime.timedelta object to travel.

To avoid potential abuse this feature is blocked when already traveling, since the shift API would provide similar functionality and should be simpler.

This should open an API to implement #247

fixes #38

[adamchainz]

Thanks for working on this.

Please add docs and a release note. (And whenever opening PR's in open source.)

[AgDude]

@adamchainz I think your concerns have been addressed. Please clarify if I am still missing something.

[adamchainz]

Yea sure, no need to comment, I’ll get around to reviewing at some point. Busy with life stuff atm.

[adamchainz]

Thanks for this. I merged this with a few changes. I decided that blocking use of timedelta within a travel() block wasn't worth the complexity.

#247 was fixed by calling the existing shift() method, but this is still useful for cases like "do a thing, then move a bit into the future and assert some state changed".

Released in 2.13.0.