Use a timedelta for the deadline setting

[Zac-HD]

The deadline setting is given as a number of milliseconds, but users could mistake this number for seconds or microseconds. Fortunately, Python has a timedelta type which can easily express durations - it would be good to accept timedeltas as values for the deadline setting, and convert numeric deadlines to timedeltas for unambiguous display.

That would mean:

• Modifying the hypothesis._settings._validate_deadline function to accept timedeltas as well as numbers, and to return a timedelta
• Make the default value for the deadline setting a timedelta
• Calculate runtime in hypothesis/core.py as a timedelta, instead of a number of milliseconds, so that the comparison works (ctrl-f for deadline to find this)
• Add a test in hypothesis-python/tests/cover/test_settings.py that passes a timedelta as a deadline, to check that it works and get full coverage
• [optionally] create a subclass of datetime.timedelta that gives the __repr__ in terms of milliseconds, and use it in hypothesis._settings._validate_deadline (only). e.g.:

class timedelta_ms(datetime.timedelta):
    def __repr__(self):
        t = self.total_seconds() * 1000
        if t == int(t):
            return "timedelta(milliseconds=%d)" % int(t)
        return datetime.timedelta.__repr__(self)

[dtfrancisco]

I would like to take a shot at this.

[Zac-HD]

Go for it! Per our usual policy, we'll reserve this issue for you for the next week 😄

Make sure you check the contributing guide - you'll need to add your name to the list of authors, and add a RELEASE.rst file, then go through the points above. If in doubt, open a work-in-progress pull request and I'll be happy to review it and help out!