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ARROW-8699: [R] Fix automatic r_to_py conversion

This appears to be the fix for https://github.com/rstudio/reticulate/issues/748

cc @kevinushey

Closes #7102 from nealrichardson/r-to-py

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>