Fixed redirect with python substitution

[anki-code]

Before

xonsh --no-rc
f = p'/tmp/file.tmp'
echo 'abc' > @(f)
# returns error: xonsh: ['/tmp/file.tmp']: unable to open file

After

xonsh --no-rc
f = p'/tmp/file.tmp'
echo 'abc' > @(f)
cat @(f)
# abc

Closes #5420

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