fix: in py3.7, as grey typo is not only a warning but error

[Calvin-Huang]

self = <imageio.plugins.pillow.JPEGFormat.Reader object at 0x123399550>, format = <Format JPEG-PIL - JPEG (ISO 10918)>
request = <imageio.core.request.Request object at 0x1233999d0>

    def __init__(self, format, request):
        self.__closed = False
        self._BaseReaderWriter_last_index = -1
        self._format = format
        self._request = request
        # Open the reader/writer
>       self._open(**self.request.kwargs.copy())
E       TypeError: _open() got an unexpected keyword argument 'as_grey'

/usr/local/lib/python3.7/site-packages/imageio/core/format.py:221: TypeError

as_gray would be a fatal error in python 3.7.