Strip type annotations from generated test files in Otter Assign

[chrispyles]

Since the exception-based test files generated by Otter Assign don't import anything except for the test_case decorator, any type annotations using the typing library or others will throw a NameError when executing the file.

def test_1(arr: np.ndarray):
    assert arr.shape == (3, 2)

Instead, Otter Assign should strip type annotations from the code it writes into these test files.