Python API changes from community

[texodus]

• Added a top-level default GLOBAL_SERVER and GLOBAL_CLIENT, and export GLOBAL_CLIENT methods as module-level free functions, similar to Perspective 2.x. Moved Server-instantiating functions like PerspectiveWidget to use GLOBAL_SERVER by default instead.
• Removed export of non-constructor Perspective types, as apparently the concept of types is scary and confusing to Python programmers.
• Moved new_local_client() and from_server methods to portable implementations.
• pyarrow and pandas are now detected via sys.modules import detection and won't be loaded unless the Python interpreter has already cached these modules. T
• Removed top-level imports of handlers packages. e.g. from perspective.handlers.tornado import PerspectiveTornadoHandler must be used explicitly now.
• Removed these re-exports from perspective.handlers as well, and the try/except block which protected lack of these modules in the dependency environment. import perspective.handlers.tornado is now a hard error without tornado installed.
• Removed top-level PerspectiveWidget import, e.g. from perspective.widget import PerspectiveWidget must be used explicitly now.
• ipywidgets is no longer a conditional import, and import perspective.widget will hard fail if this module is not installed.
• Removed Jinja2 dependency
• Added jupyter, tornado, aiohttp and starlette optional dependency bundles to pyproject.toml. pyarrow and pandas are not included as optional dependencies.
• Added View.to_dataframe() method.
• Fixes user docs.
• Fixed JSON columns reader not respecting index.
• Fixed Python sdist.