Incorrect type annotation on chain_future method

[arnaudsjs]

Problem description:

The arguments of the tornado.concurrent.chain_future method are annotated with the asyncio.Future type. The documentation and implementation of this method on the other hand indicates it also supports concurrent.futures.Future instances. This way mypy raises a type error when a concurrent.futures.Future instances is used.

Reproductive example:

# test/__init__.py
from concurrent.futures import Future
from tornado import concurrent as tornado_concurrent

f1: Future = Future()
f2: Future = Future()
tornado_concurrent.chain_future(f1, f2)

Mypy output:

$ mypy -p test

test/__init__.py:7: error: Argument 1 to "chain_future" has incompatible type "concurrent.futures._base.Future[Any]"; expected "asyncio.futures.Future[<nothing>]"  [arg-type]
test/__init__.py:7: error: Argument 2 to "chain_future" has incompatible type "concurrent.futures._base.Future[Any]"; expected "asyncio.futures.Future[<nothing>]"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Expected behavior:

The chain_future() method has a type annotation that also accepts concurrent.futures.Future instances and mypy doesn't raise a type error.