Update INSTALL.md

[nivaas219]

Python 2.7 reached End of Life on January 1, 2020. Removing it from the build dependencies to avoid confusion. Python 3 is the only supported version.

[rustbot]

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[Mark-Simulacrum]

We still support (AFAIK) Python 2.7, so I'd prefer to leave this as is. I don't think or adds much confusion here.

(Note that 2.7 reached EOL long ago, but that doesn't stop us from supporting it, even if we mostly don't use it).