Add anywidget as a hard dependency

[cpsievert]

Important projects like altair and plotly now use anywidget to implement their 2-way Jupyter bindings, but don't make anywidget a hard requirement.

That means, if you want to use altair or plotly with shinywidgets, you need to additionally install anywidget.

Since anywidget is relatively low dependency, it seems worth adding as a dependency, so users don't need to remember the additional install