Make timeout duration unit self-documenting

In a statement like @mark.timeout(50) there is nothing to say the duration is defined in seconds. This has a few disadvantages:

• Many other timing mechanisms use milliseconds or minutes as their units, so we can't simply assume that it's seconds. A new or returning user has to read the documentation (or code) to ascertain this, wasting time and producing cognitive overhead.
• Extreme timeouts like 1200 or 0.00005 have more cognitive overhead, since converting to the "natural" unit of minutes/microseconds etc isn't always trivial.

It would be nice if it instead were self-documenting. Some alternatives:

• @mark.timeout(seconds=50) would make it pretty obvious, and would make it easy to support several units.
• @mark.timeout(timedelta(minutes=5)) would make it more explicit, while supporting the widest possible range of units at no extra cost.
• @mark.timeout(seconds=timedelta(minutes=5).total_seconds()) is a clunky workaround.

Start by reviewing the current @mark.timeout API and the documentation that defines its duration units. Compare the proposed keyword-argument and timedelta interfaces, including compatibility and supported units. Done means selecting and specifying a self-documenting API, with its behavior and migration implications clearly documented.