added support for RGBA pixel matching

[novel-yet-trivial]

Sometimes the _screenshot_osx() function returns a image in rgba mode. I don't have a mac, but this was seen by someone I was helping on /r/learnpython, who was using El Capitan 10.6.8 and Python 3.5.1.

This update checks for the number of arguments the screenshot function returns, and adds the comparison of the alpha value if both the screenshot and the test value include it.

Please test; I don't have a mac and can't test it well.

[feigaoxyz]

confirm this problem

In [6]: pyscreeze.screenshot().getpixel((1200, 500))
Out[6]: (53, 75, 82, 255)

Which leads to a problem in pyautogui as well:

In [23]: pyautogui.pixelMatchesColor(1620, 650, (5, 40, 51))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-23-fd3781ddc9da> in <module>()
----> 1 pyautogui.pixelMatchesColor(1620, 650, (5, 40, 51))

/Users/user/.local/opt/pyenv/versions/miniconda3-latest/lib/python3.6/site-packages/pyscreeze/__init__.py in pixelMatchesColor(x, y, expectedRGBColor, tolerance)
    403
    404 def pixelMatchesColor(x, y, expectedRGBColor, tolerance=0):
--> 405     r, g, b = screenshot().getpixel((x, y))
    406     exR, exG, exB = expectedRGBColor
    407

[NinjaOnRails]

I had the same problem. novel-yet-trivial's solution fixed it for me. I'm on macOS Sierra running python 3.6.1

[asweigart]

Merged this and it will be part of the next release.