Empty output when calling Snap command

• uvloop version: 0.16.0
• Python version: 3.8.10
• Platform: Ubuntu 20.04.3
• Helm version: v3.7.0
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: Yes
• Does uvloop behave differently from vanilla asyncio? How?: When calling helm version via asyncio.create_subprocess_exec, vanilla asyncio displays the correct result, while uvloop returns empty stdout.

Steps to reproduce:

Download Helm CLI

curl -fsSL -o get_helm.sh https://raw.githubusercontent.com/helm/helm/main/scripts/get-helm-3
chmod 700 get_helm.sh
./get_helm.sh --version v3.7.0

Run the following Python script

import uvloop

async def _async_helm():
    proc = await asyncio.create_subprocess_exec('helm', 'version', stdout=asyncio.subprocess.PIPE)
    stdout, _ = await proc.communicate()
    print(stdout)

asyncio.run(_async_helm())
# Prints b'version.BuildInfo{Version:"v3.7.0", GitCommit:"eeac83883cb4014fe60267ec6373570374ce770b", GitTreeState:"clean", GoVersion:"go1.16.8"}\n'

uvloop.install()

asyncio.run(_async_helm())
# Prints b''

从使用 asyncio.create_subprocess_exec 的 Python 复现程序开始，并在 Ubuntu 20.04 上比较使用和不使用 uvloop.install() 时的输出。调查负责捕获 stdout 的子进程路径；当通过 uvloop 调用 helm version 返回与 vanilla asyncio 相同的非空 stdout 时，即视为完成。