Require users to set `python_version` in cog.yaml

[bfirsh]

This will stop models from breaking when we update our default.

[zeke]

To clarify, are you suggesting that python_version should be a required property in cog.yaml?

I think you also mentioned something about wanting to rename this property to python, right? If so, it might be good to roll up these two changes together.

[bfirsh]

Yes, that's right. Not sure if renaming is the right thing to do, but if it is, that sounds good.

Some of these issues are hastily written with a bunch of assumed knowledge -- sorry. Feel free to edit the main issue if they need clarification. Sometimes I treat the main issue a bit like a wiki, and add a note to the issue to say it was written by / updated by multiple people: #29

[zeke]

Feel free to edit the main issue if they need clarification

Updated the issue title. 👍🏼

[mcam10]

I think we can close this issue out?