New setting to silence warning if working inside a virtual environment
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Previously, when starting IPython in a virtual environment without IPython installed (so IPython from the global environment is used), the following warning was printed:

    Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.

This warning can be permanently silenced by setting ``c.InteractiveShell.warn_venv`` to ``False`` (the default is ``True``).