I'm trying to use dwdiff (https://os.ghalkes.nl/dwdiff.html) to compare two text files. While running subprocess I'm getting the following errors:

```
Traceback (most recent call last):
  File "test_dwdiff.py", line 17, in <module>
    asyncio.run(main())
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 579, in run_until_complete
    return future.result()
  File "test_dwdiff.py", line 14, in main
    await asyncio.gather(*(dwdiff() for __ in range(25)))
  File "test_dwdiff.py", line 10, in dwdiff
    await process.communicate()
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/subprocess.py", line 187, in communicate
    loop=self._loop)
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/subprocess.py", line 166, in _read_stream
    output = await stream.read()
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/streams.py", line 633, in read
    block = await self.read(self._limit)
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/streams.py", line 646, in read
    self._maybe_resume_transport()
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/streams.py", line 417, in _maybe_resume_transport
    self._transport.resume_reading()
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/unix_events.py", line 498, in resume_reading
    self._loop._add_reader(self._fileno, self._read_ready)
AttributeError: 'NoneType' object has no attribute '_add_reader'
```

and

```
Exception in callback SubprocessStreamProtocol.pipe_data_received(1, b'\xd0\x9f\xd...b8\xd1\x8f.\n')
handle: <Handle SubprocessStreamProtocol.pipe_data_received(1, b'\xd0\x9f\xd...b8\xd1\x8f.\n')>
Traceback (most recent call last):
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/subprocess.py", line 71, in pipe_data_received
    reader.feed_data(data)
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/streams.py", line 440, in feed_data
    self._transport.pause_reading()
  File "/home/decaz/.pyenv/versions/3.7.4/lib/python3.7/asyncio/unix_events.py", line 495, in pause_reading
    self._loop._remove_reader(self._fileno)
AttributeError: 'NoneType' object has no attribute '_remove_reader'
```

The code is the following:
```
import asyncio

async def dwdiff():
    process = await asyncio.create_subprocess_exec(
        '/usr/local/bin/dwdiff', 'f1.txt', 'f2.txt',
        stdout=asyncio.subprocess.PIPE,
        stderr=asyncio.subprocess.PIPE,
    )
    await process.communicate()

async def main():
    await asyncio.gather(*(dwdiff() for __ in range(25)))

asyncio.run(main())
```

Two text files f1.txt and f2.txt are attached as archive.

Perhaps it relates to https://bugs.python.org/issue29704

Thanks. The correct fix is 'do nothing if stream._transport is None'.
We have only two weeks before 3.8rc, thus I'll prepare a fix myself for the sake of fast merging.

This fix cannot land on 3.6, the version is in security-fix only mode.

Andrew, it would be nice to have it fixed within 3.8. Hope you'll find time for this.

New changeset 58498bc7178608b1ab031994ca09c43889ce3e76 by Andrew Svetlov in branch 'master':
bpo-38019: correctly handle pause/resume reading of closed asyncio unix pipe (GH-16472)
https://github.com/python/cpython/commit/58498bc7178608b1ab031994ca09c43889ce3e76

New changeset 1c3e4691bb41fa070d6f4836cb03005123e53e4b by Miss Islington (bot) in branch '3.7':
bpo-38019: correctly handle pause/resume reading of closed asyncio unix pipe (GH-16472)
https://github.com/python/cpython/commit/1c3e4691bb41fa070d6f4836cb03005123e53e4b

New changeset 19cd5951ec4480c7cfcbcc379b36902312cfb8b8 by Miss Islington (bot) in branch '3.8':
bpo-38019: correctly handle pause/resume reading of closed asyncio unix pipe (GH-16472)
https://github.com/python/cpython/commit/19cd5951ec4480c7cfcbcc379b36902312cfb8b8

Andrew, thank you!