Add Python client implementation

[sc1f]

This PR introduces PerspectiveClient, which mimics the Javascript client implementation by offering a fully async (asyncio or yield-based) API on top of the core Table and View public API. This is useful for testing perspective-python servers, as it offers programmatic access to Perspective's public API in Python.

PerspectiveTornadoClient is an implementation of the client API that allows users to open up remote Tables and Views on a perspective-python server, and treat them as if they were objects in the client runtime using either await or yield.

The interface is extremely simple, using async and await:

import asyncio
from perspective.tornado_handler import websocket

current_loop = asyncio.get_event_loop()
ws = websocket("ws://.../", ioloop=current_loop)

async def run():
    table = ws.open_table("data_source")
    schema = await table.schema()
    print(schema)  # {"a": "float"}
    view = table.view(row_pivots=["a"])
    data = await view.to_arrow()
    print(type(arrow)) # bytes

current_loop.run_until_complete(run())  # or current_loop.call_soon(run)

or yield-based coroutines:

from tornado import gen
from tornado.ioloop import IOLoop
from perspective.tornado_handler import websocket

current_loop = IOLoop.current()
ws = websocket("ws://.../", ioloop=current_loop)

@gen.coroutine
def run():
    table = ws.open_table("data_source")
    schema = yield table.schema()
    print(schema)  # {"a": "float"}
    view = table.view(row_pivots=["a"])
    data = yield view.to_arrow()
    print(type(arrow)) # bytes
current_loop.run_sync(run)  # or current_loop.add_callback(run)

[texodus]

Thanks for the PR! Looks great and is a super useful addition! There is a small bug I think with table names, but I will take care of fixing these in a cleanup PR.

[texodus]

The default argument to name here is evaluated once when the method is created, so all proxy tables will have the same name.

[sc1f]

yup - that is a silly mistake on my part

[texodus]

Same, and also it's probably a good idea to just make name a required arg here as this table is private to the library.

[texodus]

Unrelated - but I think we can deprecate this "feature flag" - its implementation returns true for JS and Python

[texodus]

Empty file

[texodus]

Only adds commented blocks

[texodus]

Also - I believe the FOSSA check is either malfunctioning or has recently received an update to its detection criteria. This PR specifically does not introduce dependencies, so if there are legitimate failures, they are pre-existing regardless. Thus, I'm going to override this check.