Fix for ColorClip.rotate()

[tburrows13]

Fixes #1140.

Previously, attempting to use clip.rotate(...) when clip is a ColorClip would crash:

TypeError: Cannot handle this data type: (1, 1, 3), <i8

Traceback (most recent call last):
  File "/Users/tomburrows/miniconda3/envs/moviepy/lib/python3.7/site-packages/PIL/Image.py", line 2680, in fromarray
    mode, rawmode = _fromarray_typemap[typekey]
KeyError: ((1, 1, 3), '<i8')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/tom_t.py", line 64, in <module>
    c = c.rotate(lambda t: 45 + 3*t)
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/video/fx/rotate.py", line 84, in rotate
    return clip.fl(fl, apply_to=["mask"])
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/video/VideoClip.py", line 1000, in fl
    newclip = VideoClip.fl(self, fl, apply_to=apply_to, keep_duration=keep_duration)
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/Clip.py", line 140, in fl
    newclip = self.set_make_frame(lambda t: fun(self.get_frame, t))
  File "</Users/tomburrows/miniconda3/envs/moviepy/lib/python3.7/site-packages/decorator.py:decorator-gen-61>", line 2, in set_make_frame
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/decorators.py", line 14, in outplace
    f(newclip, *a, **k)
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/video/VideoClip.py", line 721, in set_make_frame
    self.size = self.get_frame(0).shape[:2][::-1]
  File "</Users/tomburrows/miniconda3/envs/moviepy/lib/python3.7/site-packages/decorator.py:decorator-gen-11>", line 2, in get_frame
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/decorators.py", line 87, in wrapper
    return f(*new_a, **new_kw)
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/Clip.py", line 97, in get_frame
    return self.make_frame(t)
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/Clip.py", line 140, in <lambda>
    newclip = self.set_make_frame(lambda t: fun(self.get_frame, t))
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/video/fx/rotate.py", line 82, in fl
    return pil_rotater(im, a, resample=resample, expand=expand)
  File "/Users/tomburrows/Python/moviepy-dev/moviepy/moviepy/video/fx/rotate.py", line 12, in pil_rotater
    Image.fromarray(pic).rotate(angle, expand=expand, resample=resample)
  File "/Users/tomburrows/miniconda3/envs/moviepy/lib/python3.7/site-packages/PIL/Image.py", line 2682, in fromarray
    raise TypeError("Cannot handle this data type: %s, %s" % typekey)
TypeError: Cannot handle this data type: (1, 1, 3), <i8

Discovered on Stack Overflow: https://stackoverflow.com/questions/60940266

Explanation: when a ColorClip is created (for example c = ColorClip([600, 400], color=[150, 200, 100])), each pixel is stored as whatever is passed through as the color parameter. However, Image.fromarray() (from pillow) requires each pixel to be in the "uint8" format. When writing to a file, this is performed in Clip.iter_frames(), so this PR simply calls .astype("uint8") inside of rotate().

• 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 formatted my code using black -t py36

[coveralls]

Coverage decreased (-0.4%) to 64.352% when pulling 6cebb97 on tburrows13:fix-colorclip-rotate into 4106b4c on Zulko:master.

[tburrows13]

I think that this is ready to merge.