Allow passing dt.timedelta objects to travel()

[adamchainz]

It would offset the current time by the given amount, which would be pretty useful for some tests that check "X is true until N minutes in the future"

[soxofaan]

Hi, I just wanted to check if is there still interest or momentum to move any of these related/duplicate issues forward:

• Allow passing dt.timedelta objects to travel() #38
• Add shift to TimeMachineFixture #247
• Add support for dt.timedelta to travel class #298
• Add shift() method to time_machine pytest fixture #312

I'm currently working around the limitations, but it would be nice to know if there will be some cleaner approach with a future release of time_machine