Datasette(... files=) should not be a required argument

[simonw]

>>> ds = Datasette(memory=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'files'
>>> ds = Datasette(memory=True, files=[])

I wanted to create an in-memory Datasette for running some tests, no point in forcing me to pass files=[] to do that.

[simonw]

I should document the usage of this constructor in https://docs.datasette.io/en/stable/internals.html#datasette-class

[simonw]

Docs here: https://docs.datasette.io/en/latest/internals.html#datasette-class