Issue 34262: Asyncio test fails under Win 7

When running tests I encounter the following error for the test:

test_sock_sendfile_not_regular_file (test.test_asyncio.test_proactor_events.ProactorEventLoopUnixSockSendfileTests)

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Microsoft Visual C++ Runtime Library
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Debug Assertion Failed!

Program: ...\EP2018\cpython\PCbuild\win32\python_d.exe
File: minkernel\crts\ucrt\src\appcrt\lowio\osfinfo.cpp
Line: 257

Expression: fh >= 0 && (unsigned)fh < (unsigned)_nhandle


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However, when ignoring the MSCR error, the tests succeed without failing.

In addition the following failed (but not consistently):

======================================================================
FAIL: test_sendfile_close_peer_in_the_middle_of_receiving (test.test_asyncio.test_events.ProactorEventLoopTests)
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Traceback (most recent call last):
  File "C:\Users\woellert\Desktop\EP2018\cpython\lib\test\test_asyncio\test_events.py", line 2510, in test_sendfile_close_peer_in_the_middle_of_receiving
    self.file.tell())
AssertionError: False is not true : 131072

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Specs: Windows 7 SP1 64bit, 

Process:
- Cloned master from github
- Installed Visual Studio 2017 Community
- Compiled with `PCbuild\build.bat -e -d` (also used 64bit via -p x64)
- Ran tests with `python.bat -m test.test_asyncio -v`

Regarding the MVSC runtime message, there is an issue already here:

https://bugs.python.org/issue23919

ProactorEventLoop is the default event loop with issue34687. There doesn't seem to be any warnings in buildbots as I have checked and as @panesen has mentioned this is not consistent. I am adding Victor as a notification if he has any thoughts. Victor, feel free to remove yourself if this is not relevant.

Thanks

Changing type since the crash was fixed under issue23919.

There remains (possibly) the failure of test_sendfile_close_peer_in_the_middle_of_receiving.

test_sendfile_close_peer_in_the_middle_of_receiving() failure is tracked by bpo-41682.

messages: + msg401282
nosy: + iritkatriel