avoid self._errpipe_write double close

[graingert]

Fixes #414

the I couldn't get the original MRE from the issue https://gist.github.com/decorator-factory/6b5276ab5fb87f4c009f1d22dc4b20a9 to always fail - it seems to be intermittent. However the following always fails for me without this patch:

import os
import asyncio
import subprocess

import uvloop
uvloop.install()


async def main():
    done = False
    def preexec_fn():
        pass

    async def fork():
        nonlocal done
        try:
            for i in range(1000):
                await asyncio.sleep(0)
                try:
                    proc = await asyncio.create_subprocess_exec("true", preexec_fn=preexec_fn)
                    await proc.communicate()
                except subprocess.SubprocessError:
                    print("done")
        finally:
            done = True

    def write_file():
        while not done:
            with open("demo", "w") as f:
                f.write("hello")
                print(f.fileno())

    await asyncio.gather(
        fork(),
        asyncio.to_thread(write_file)
    )

asyncio.run(main())

[graingert]

the write_pipe can be closed again sometimes:

uvloop/uvloop/handles/process.pyx

Lines 118 to 122 in 3e71ddc

	try:
	os_close(self._errpipe_write)
	except OSError:
	# Might be already closed
	pass

if another thread allocates a file descriptor in this time then instead of raising OSError here the fd will be closed incorrectly

[graingert]

@fantix can I get a review of this please?

[fantix]

This is good; I was trying to write a test but it's going to take some time, let's merge this first.