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/building.rst

@@ -45,13 +45,13 @@ Prerequisites
 
 You will need the following:
 
-- `Python 2.7 <https://www.python.org/downloads/windows/>`_
+- `Python 3.x <https://www.python.org/downloads/windows/>`_
 - A copy of the `Pillow sources <https://github.com/python-pillow/Pillow>`_.
 - The Pillow Extension for Python.
 - The Numpy Extension for Python.
 - The extensions can be installed via::
 
-    c:\python27\python.exe -m pip -U numpy pillow
+    c:\python37\python.exe -m pip -U numpy pillow
 
 
 Building with Visual Studio
@@ -63,7 +63,7 @@ Building with Visual Studio
 4. Copy Imaging.h and ImPlatform.h from your Pillow sources into the current working directory.
 5. First try a build::
 
-    c:\python27\python setup.py build
+    c:\python37\python setup.py build
 
 If you encounter the following errors::
 
@@ -73,7 +73,7 @@ then try the following::
 
     set DISTUTILS_USE_SDK=1
     set MSSdk=1
-    c:\python27\python setup.py build
+    c:\python37\python setup.py build
 
 If the build was successful, there should be a c_overviewer.pyd file in your current working directory.
 
@@ -145,7 +145,7 @@ macOS
 -----
 
 1. Install xCode Command Line Tools by running the command (``xcode-select --install``) in terminal (located in your /Applications/Utilities folder
-2. Install Python 2.7.10 if you don't already have it https://www.python.org/ftp/python/2.7.10/python-2.7.10-macosx10.6.pkg
+2. Install Python 3 if you don't already have it, for example from `the official Python website <https://www.python.org/downloads/mac-osx/>`_.
 3. Install PIP (``sudo easy-install pip``)
 4. Install Pillow (overviewer needs PIL, Pillow is a fork of PIL that provides the same funcitonality) (``pip install Pillow``)
 5. Download the Pillow source files from https://github.com/python-pillow/Pillow/releases/latest and unpack the tar.gz file and move it to a directory you can remember
@@ -153,19 +153,8 @@ macOS
 7. Extract overviewer-[Version].tar.gz and move it to a directory you can remember
 8. Go into your Pillow-[Version] folder and navigate to the /src/libImaging directory
 9. Drag the following files from the Pillow-[Version]/src/libImaging folder to your overviewer-[Version] folder (``Imaging.h, ImagingUtils, ImPlatform.h``)
-10. Symlink Python by running the command (``sudo ln -sf /usr/bin/python2.7 /usr/local/bin/python2``) in terminal
+10. Make sure your installation of Python 3 is in ``$PATH``
 11. In terminal change directory to your overviewer-[Version] folder (e.g ``cd Desktop/overviewer-[Version]``)
 12. Build::
 
-    (``PIL_INCLUDE_DIR="/path/to/Pillow-[version]/libImaging" python2 setup.py build``)
+    (``PIL_INCLUDE_DIR="/path/to/Pillow-[version]/libImaging" python3 setup.py build``)
-
-FreeBSD
--------
-FreeBSD is similar to macOS and Linux, but ensure you're using Python 2.7. The port of Python 2.6 has bugs with threading under FreeBSD.
-Everything else you should need is ported, in particular math/py-numpy and graphics/py-imaging.
-
-You may need or want to add the line::
-
-    PYTHON_VERSION=2.7
-
-to the file /etc/make.conf, but read the ports documentation to be sure of what this might do to other Python applications on your system.  

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

docs/index.rst

@@ -116,7 +116,7 @@ Requirements
 This is a quick list of what's required to run The Overviewer. It runs on
 Windows, Mac, and Linux as long as you have these software packages installed:
 
-* Python 2.6 or 2.7 (we are *not* yet compatible with Python 3.x)
+* Python 3.4 or above (we are no longer compatible with Python 2.x)
 
 * PIL (Python Imaging Library) or Pillow