Fix requirements parsing and declare Python >= 3.9

[SY-Jia06]

Motivation and Context

setup.py previously split requirements.txt verbatim, which includes comment lines. Those can be interpreted as invalid requirements during pip install -e habitat-lab. This change filters comments/blank lines, updates the Python badge to 3.9, and declares python_requires>=3.9 for consistent packaging metadata.

How Has This Been Tested

Not run (packaging metadata only).

Types of changes

• [Bug Fix] (non-breaking change which fixes an issue)
• [Docs change] (badge update)

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[SY-Jia06]

Small packaging fix, ready for review. No runtime changes.