Document how to install the correct Python version for development

[pietervdvn]

The script first-setup uses .python-version which is very strict. Not many people will have precisely this version.

I'd propose to change 3.9.2 into 3.9 or even 3 as to allow an easier setup.

[redfast00]

Python is not forwards-compatible or semantically versioned (source: https://stackoverflow.com/a/60422284/5431090, this is a great read), so if a developer is on a slightly different version of Python, code might work on their machine, but not in production. To avoid this, we pin the version of all dependencies (so both the language itself and the libraries we use).

I do understand that most people won't have that exact version of Python on their machine (and some might even break their OS trying to remove Python and then have to install NixOS 😉 ). I manage my programming runtimes with asdf and find it to be very easy to use. It has support for a ton of languages and really removes the headache of managing versions of runtimes, so I recommend giving it a try.

[pietervdvn]

Oh, didn't expect python to be non-semantically versioned...

In that case, it might be good to document how to setup the specific python version in the README. I'm the kind of person who doesn't like to tamper with these things, at it'll make onboarding easier.

PS: NixOS is amazing ;)