`loop.run_until_complete` raises `RuntimeError` after catching `SystemExit`

[Jackenmen]

• uvloop version: 0.14.0
• Python version: 3.8.2
• Platform: Ubuntu 18.04
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: Yes
• Does uvloop behave differently from vanilla asyncio? How?: Yes, vanilla asyncio doesn't raise for this code

Reproduction code:

import asyncio
import sys
import uvloop

asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
loop = asyncio.new_event_loop()

async def x():
    sys.exit(1)

return_code = 0
try:
    loop.run_until_complete(x())
except SystemExit as exc:
    return_code = exc.code
finally:
    loop.run_until_complete(asyncio.sleep(1))
    sys.exit(return_code)

Traceback:

Traceback (most recent call last):
  File "test.py", line 17, in <module>
    loop.run_until_complete(asyncio.sleep(1))
  File "uvloop/loop.pyx", line 1454, in uvloop.loop.Loop.run_until_complete
RuntimeError: Event loop stopped before Future completed.

Same happens when KeyboardInterrupt is raised - vanilla asyncio handles both of these here: https://github.com/python/cpython/blob/master/Lib/asyncio/base_events.py#L187-L190