TypeError('cancel() takes exactly one argument (2 given)') with py3.9.0rc2

[mgorny]

• uvloop version: 6ef69a7 (also 0.14.0)
• Python version: 3.9.0rc2
• Platform: Gentoo Linux
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?:
• Does uvloop behave differently from vanilla asyncio? How?: my educated guess is that asyncio doesn't crash on itself ;-)

Running the test suite under Python 3.9 gives a number of errors due to wrong cancel() arguments, e.g.:

$ PYTHONPATH=../build/lib.linux-x86_64-3.9/ python3.9 -m unittest test_sockets.TestUVSockets.test_sock_cancel_add_reader_race
/tmp/uvloop/tests/test_sockets.py:225: DeprecationWarning: The explicit passing of coroutine objects to asyncio.wait() is deprecated since Python 3.8, and scheduled for removal in Python 3.11.
  _, pending_read_futs = await asyncio.wait(
Task was destroyed but it is pending!
task: <Task pending name='Task-4' coro=<_TestSockets.test_sock_cancel_add_reader_race.<locals>.client.<locals>.send_server_data() done, defined at /tmp/uvloop/tests/test_sockets.py:229> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x7fc15eabf0a0>()]>>
E
======================================================================
ERROR: test_sock_cancel_add_reader_race (test_sockets.TestUVSockets)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/uvloop/tests/test_sockets.py", line 246, in test_sock_cancel_add_reader_race
    self.loop.run_until_complete(server())
  File "uvloop/loop.pyx", line 1477, in uvloop.loop.Loop.run_until_complete
  File "/tmp/uvloop/tests/test_sockets.py", line 218, in server
    await fut
  File "/tmp/uvloop/tests/test_sockets.py", line 240, in client
    rfut.cancel()
TypeError: cancel() takes exactly one argument (2 given)

----------------------------------------------------------------------
Ran 1 test in 1.020s

FAILED (errors=1)