vfs: modernize the detection of the main thread
There weren't a lot of good choices when py27 was supported, but starting with
py34, `threading.main_thread()` is available. This gets us away from an
undocumented, internal symbol, and drops a pytype suppression statement. It is
also apparently no longer reliable after a process fork.[1][2]
[1] https://stackoverflow.com/a/23207116
[2] https://github.com/python/cpython/blob/v3.6.3/Lib/threading.py#L1334
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