Fix issue #2286 on TimeMirror and TimeSymmetrize

[OsaAjani]

Reversing a clip by slicing was removing last second of a clip, as described by #2286, this was due to an error in the implementation of Clip.__getitem__() where we was subtracting 1 instead of 1/self.fps. This was not showing in unit test because unit test was using a 1FPS video.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it