[PDDF] Fix missing import on pddf_thermal.py

[seanwu-ec]

Signed-off-by: Sean Wu sean_wu@edge-core.com

Why I did it

To fix NameError exception while set_high_threshold() get invoked.

File "/usr/local/lib/python3.7/dist-packages/sonic_platform_pddf_base/pddf_thermal.py", line 97, in set_high_threshold
    os.system(cmd)
NameError: name 'os' is not defined

How I did it

import os

How to verify it

1. The exception is gone.
2. Use sensors command to confirm the threshold was changed after set_high_threshold().

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012

Description for the changelog

Fix missing import on pddf_thermal.py

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