Download of new Python versions

[DiTo97]

After experimenting a bit with adding Python versions on Travel, it probably should be tweaked a bit, mainly because it forces you to always have that version of Python in the system, which can be somewhat limiting (e.g., we need a specific image, say in a step of a Bitbucket pipeline, but it doesn't have the right Python version)

I suggest using python-build, a Pyenv plugin that takes care of the whole installation procedure. As you can see from the README, it does not need Pyenv to be installed, but can operate standalone like this:

python-build <python-version> <output-directory>

where <python-version> is the version of Python to install (e.g., 3.7.15) and <output-directory> is where to install it.

It should be quite easy (maybe with Git submodules?) to go and reference Pyenv's plugin directly, so you can keep it up to date, and at the same time change the Travel command to add Python versions to something like:

1. ​travel config add python <python-version> [<python-location>]​
2. If the Python version has already been added, ignore the command (or add a flag to force the override)
3. Is python-location available?
   • If yes, look for the Python binary from there and:
     • if it is not present, raise a FileNotFoundError
     • if it is present, but the binary does not match <python-version>, raise a ValueError
   • If no, run python-build on that version to download the binary to some known internal path
4. Add that (version, binary) pair to Travel config.yml

N.B.: The above flow chart works only on UNIX-like systems. For Windows systems, we should look for an equivalent standalone plugin in Pyenv-win.