docs: move last mentions of Python 2 to Python 3

I've adjusted the macOS instructions slightly to be more general,
e.g. do not advise symlinking a specific Python executable of a
specific version, just tell people they should make sure it's in
their environment.

And yes, on Windows, python.exe is whatever version you have installed,
so if you install Python 3, python.exe is going to be Python 3.

I've removed the FreeBSD instructions because they only talk about
working around ports potentially using Python 2.6, which is no longer
relevant. If somebody here uses FreeBSD and has anything to add to the
instructions not already covered by the Linux instructions, let me know.

docs/contributing.rst

@@ -13,7 +13,7 @@ changes back into the project, for help on how to compile the Overviewer, check
 Prerequisites
 =============
 
-Ideally you're familiar with Python (Overviewer uses Python 2), and know the
+Ideally you're familiar with Python (Overviewer uses Python 3), and know the
 basics of Git. Both have various very good resources online that help you in
 learning them, but the best way of learning is always to use them in the real
 world, so don't hesitate to jump right in after having a basic grasp and ask