Add note about why 3.9.3 omitted

[jeffwidman]

Add note about why 3.9.3 excluded

[jeffwidman]

Here's the commit that added this: e43c80f

So when this note was added, the vulnerable versions were in the 3.5.x series... 3.6 is now EOL from upstream Python, although we're unofficially made a number of choices to continue to support it until Ubuntu 18.04 goes EOL this coming April.

Before that, there was a note about poetry not supporting python versions, which got added way back here: 6f734df#diff-fcc7191de583ab48f2202111f3076e45200c2ff6b83bb4b3f88425a81ee604e4R3-R8

So I wonder if poetry now supports this?? In which case we could drop this hardcoded python version list altogether?? 🤔

[jeffwidman]

I filed #6044 to track this, as not something I have time to look into right now. For now IMO we should merge this to unblock ourselves/get correct, then later circle back and clean this up.