Consider dropping pyenv

[smangham]

• Most of the people coming with their own laptops had Conda installed, and Conda's $PATH modifications conflict with pyenv's.
• Pyenv also makes debugging which python interpreter is active harder - our previous advice about which python fails as it aliases python and python3 to a shim, so which python always returns just the shim regardless. python --version then errors because the shim is pointing at nothing (whilst python3 --version works because the shim points at system python).

It was pretty hard to answer questions on pyenv when I don't use it. Checking, I can't find a .python-version file in any of the Southampton RSG project directories, and there's only <25k across the entirety of GitHub (and that search kept throwing up "-python-version" in filenames too), so I just don't think it's common enough to justify inclusion. Given Poetry will also manage Python versions so we're already teaching a solution even for people who don't use Conda, we can probably just safely drop it.