docs: Python 3 is supported

Using `sudo` is actually a bad idea

Author: pietrodn

docs/develop/installation.rst

@@ -5,7 +5,7 @@ Installing DroneKit
 ===================
 
 DroneKit-Python can be installed on a Linux, Mac OSX, or Windows computer that 
-has *Python 2.7* and can install Python packages from the Internet.
+has Python 2.7 or Python 3 installed and can install Python packages from the Internet.
 
 It is installed from **pip** on all platforms:
 
@@ -16,23 +16,25 @@ It is installed from **pip** on all platforms:
 
 **Installation notes:**
 
-* Mac and Linux require you prefix the command with ``sudo``.
+* Install `dronekit` with `pip` inside a virtualenv:
 
   .. code-block:: bash
 
-      sudo pip install dronekit
+      python3 -m venv .venv
+      . .venv/bin/activate
+      pip install dronekit
       
 * On Linux you may need to first install **pip** and **python-dev**:
 
   .. code-block:: bash
 
       sudo apt-get install python-pip python-dev
       
-  Alternatively, the *easy_install* tool provides another way to install pip:
+  Alternatively, you can use the `ensurepip` module to install or upgrade Pip on Python 3:
 
   .. code-block:: bash
 
-      sudo easy_install pip
+      python -m ensurepip --upgrade
       
 * :doc:`companion-computers` are likely to run on stripped down versions of Linux. Ensure
   you use a variant that supports Python 2.7 and can install Python packages from the Internet.
@@ -43,4 +45,4 @@ It is installed from **pip** on all platforms:
   * `WinPython 2.7 64bit <http://sourceforge.net/projects/winpython/files/WinPython_2.7/>`_ (see 
     `these instructions for installation and registration <https://github.com/winpython/winpython/wiki/Installation>`_). This is the most tested version.    
   * `ActiveState ActivePython 2.7 <http://www.activestate.com/activepython/downloads>`_.
-* Python 3 is not supported.
+* Python 3 is fully supported.