Introduction: Update the description of Python versions

[jondo]

It currently says:

Python 3 will eventually supercede Python 2, but it is not backward-compatible with Python 2. A lot of existing python code and packages has been written for Python 2, and it is still the most wide-spread version. For these lectures either version will be fine, but it is probably easier to stick with Python 2 for now, because it is more readily available via prebuilt packages and binary installers.

I suggest updating this to

Python 2 was officially discontinued in 2020. Python 3 has superseded Python 2 in most areas, although it is not backward-compatible with Python 2. A lot of existing python code and packages that has been written for Python 2 has now been ported to Python 3. For these lectures either version will be fine, but it is probably easier to continue with Python 3.