Add the ability to load values from python files

[kalekundert]

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The basic idea would be to load the file and then to look in globals() for test cases, e.g.:

test_foo = [
    dict(
        given=1,
        expected=2,
    ),
    dict(
        given=3,
        expected=4,
    ),
    ...
]

test_bar = [
    ...
]

The trick is deciding what default file name to look for. I can't simply use the .py extension, because that would be the same name as the test file. Maybe I could use the _params.py suffix.