Shinywidgets

* Fixed a bug with multiple altair outputs not working inside a `shiny.render.ui` decorator. (140)
* `render_widget` no longer errors out when giving a `altair.FacetChart` class. (142)
* `render_widget` no longer fails to serialize `decimal.Decimal` objects. (138)

* Widgets no longer have a "flash" of incorrect size when first rendered. (133)
* `render_widget` now works properly with `Widget`s that aren't `DOMWidget`s (i.e., widgets that aren't meant to be displayed directly). As a result, you can now use `render_widget` to gain a reference to the widget instance, and then use that reference to update the widget's value. (133)

* The `render_widget` decorator now attaches a `widget` (and `value`) attribute to the function it decorates. This allows for easier access to the widget instance (or value), and eliminates the need for `register_widget` (which is now soft deprecated). (119)
* Added decorators for notable packages that require coercion to the `Widget` class: `render_altair`, `render_bokeh`, `render_plotly`, and `render_pydeck`. Using these decorators (over `render_widget`) helps with typing on the `widget` attribute. (119)
* The `.properties()` method on `altair.Chart` object now works as expected again. (129)
* Reduce default plot margins on plotly graphs.

* Fixed several issues with filling layout behavior introduced in 0.2.3. (124, 125)
* `reactive_read()` now throws a more informative error when attempting to read non-existing or non-trait attributes. (120)

* Widgets now `fill` inside of a `fillable` container by default. For examples, see the [ipyleaflet](https://github.com/posit-dev/py-shinywidgets/blob/main/examples/ipyleaflet/app.py), [plotly](https://github.com/posit-dev/py-shinywidgets/blob/main/examples/plotly/app.py), or other [output](https://github.com/posit-dev/py-shinywidgets/blob/main/examples/outputs/app.py) examples. If this intelligent filling isn't desirable, either provide a `height` or `fillable=False` on `output_widget()`. (#115)
* `as_widget()` uses the new `altair.JupyterChart()` to coerce `altair.Chart()` into a `ipywidgets.widgets.Widget` instance. (113)

* `render_widget` now builds on `shiny`'s `render.transformer` infrastructure, and as a result, it works more seamlessly in `shiny.express` mode. (110)
* Closed 104: Officially support for Python 3.7.