✨ Properly implement `pyenv` & `pipenv`

[Rikj000]

Make mgm-hurry run under:

• pyenv: Lets you easily switch between multiple versions of Python.
• pipenv: The new currently preferred to use package for Python virtual environments for dependency packages and also should be able to work together with Freqtrade's older virtualenv (Older package that does the same for freqtrade itself).

The benefit of running everything under pyenv & pipenv will be that we eliminate the Python +3.8 dependency.
(Since the virtual environment will use whichever Python version we need, stand-alone from the main Python version used by the OS).
Quite a few Linux distributions are not on Python +3.8 yet, which currently renders them unable to do a source install.

[Rikj000]

Those who wish to be early, you can already install pyenv by:

• Installing the needed build environment dependency packages
• Running the pyenv-installer
• Don't forget to add the needed lines to your shell config (e.g. ~/.bashrc, the pyenv-installer will prompt you for this at the end of it's installation)

[Rikj000]

Added in commits: 1117c45...0b3f520 🎉