Repeating calls to connect_write_pipe with sys.stdout as a stream lead to FileExistsError exception

Nobody has claimed this yet.

* **uvloop version**: 0.11.3, 0.12.2, 0.13.0rc1
* **Python version**: 3.6, 3.7
* **Platform**: linux, macos
* **Can you reproduce the bug with `PYTHONASYNCIODEBUG` in env?**: yes

Hello, this example doesn't work with uvloop.
```python
import asyncio
import sys
import uvloop

uvloop.install()
loop = asyncio.get_event_loop()

async def main():
transport, protocol = await loop.connect_write_pipe(
asyncio.Protocol, sys.stdout
)
print("okay")

transport2, protocol2 = await loop.connect_write_pipe(
asyncio.Protocol, sys.stdout
)
print("okay")

loop.run_until_complete(main())
```

```
okay
Traceback (most recent call last):
File "uvl.py", line 20, in
loop.run_until_complete(main())
File "uvloop/loop.pyx", line 1451, in uvloop.loop.Loop.run_until_complete
File "uvl.py", line 16, in main
asyncio.Protocol, sys.stdout
File "uvloop/loop.pyx", line 2703, in connect_write_pipe
File "uvloop/loop.pyx", line 2698, in uvloop.loop.Loop.connect_write_pipe
File "uvloop/handles/pipe.pyx", line 170, in uvloop.loop.WriteUnixTransport._open
File "uvloop/handles/pipe.pyx", line 29, in uvloop.loop.__pipe_open
FileExistsError: [Errno 17] File exists
```

please, see discussion here https://github.com/B2W-BIT/aiologger/issues/42#issuecomment-457283504

Reproduce the example using repeated loop.connect_write_pipe calls with sys.stdout, then inspect the stack-trace entry points in uvloop/loop.pyx and uvloop/handles/pipe.pyx. Compare the first and second pipe-opening paths and add regression coverage; done means the repeated calls complete without FileExistsError on the reported platforms.

Written by the indexing model from the issue text.