Why hadn't I read #595601 in detail, it has an explanation: 

[quoting Jeremy Hylton]
The universal newline code is squirrels the FILE * in a
local variable, which is worse.  If it happens that
another thread closes the file, at best the local
points to a closed FILE *.  But that memory could get
recycled and then there's no way to know what it points to.
[/quoting]

Even with careful coding, there's a small window between releasing the
GIL on our side, and acquiring the FILE-specific lock in the glibc,
during which the fclose() function can be invoked and release the FILE
just before we invoke another function (e.g. fseek()) on it.