#0: remove pinned python3.8-venv in docker requirements

[vtangTT]

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Checklist

• Post commit CI passes
• Blackhole Post commit (if applicable)
• Model regression CI testing passes (if applicable)
• New/Existing tests provide coverage for changes

[ttmchiou]

for documentation purposes:
Remove pinned version of python3.8-venv as it is considered (hopefully) as stable enough verison. New releases of python3.8-venv causes the older version to be released from apt repositories