Make v8py.Context a Python context manager

[tbodt]

Why is it useful for Context to be a context manager?

[tbodt]

Also, creating a context fails on Python 3:

self = <v8py.context.Context object at 0x7ff3ba7d0518>
args = (<class 'test_global.Global.<locals>.Global'>,), kwargs = {}
    def __init__(self, *args, **kwargs):
>       _v8py.Context.__init__(self, *args, **kwargs)
E       TypeError: object.__init__() takes no parameters
v8py/context.py:5: TypeError

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Context as a context manager could be useful so that it can be used directly in with statements without first having to be wrapped by contextlib.closing(). Apart from being a Pythonic approach, it would also allow to manage some initialization/destruction details transparently in the next future if needed.

[tbodt]

In v8py, contexts don't need to be opened or closed, they can just be used.