On Python <= 3.11, pathlib._NormalAccessor captures io.open (and os.* mutators)
into class attributes at pathlib import time. A C builtin captured there does not
bind on instance access, but a Python wrapper does: self shifts into the next
positional, so Path.open / Path.write_text raised 'open() argument mode must be
str, not PosixPath' once the guard had replaced io.open with a Python wrapper
before pathlib was imported. (3.12+ dropped the accessor, which is why it only
failed on the 3.10 CI leg.)
Import io and pathlib at the top of the guard, before any patching, so the
accessor captures the original builtins, and confine Path.open by wrapping the
public method directly (mode-aware) rather than relying on the io.open patch to
reach it. Direct io.open() writers are still guarded for the zipfile-based cases.
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