Added documentation about Python 3.4 and earlier now having an additional dependency on contextlib2

Author: tleonhardt

CHANGELOG.md

@@ -30,6 +30,7 @@
 * Other changes
     * The **edit** command no longer allows you to edit prior commands. The capability to edit prior commands is now part of the **history** command. The **edit** command still allows you to edit arbitrary files.
     * the **autorun_on_edit** setting has been removed.
+    * For Python 3.4 and earlier, ``cmd2`` now has an additional dependency on the ``contextlib2`` module
 * Deprecations
     * The old **options** decorator for optparse-based argument parsing is now *deprecated*
         * The old decorator is still present for now, but will be removed in a future release

README.md

@@ -48,8 +48,9 @@ pip install -U cmd2
 
 cmd2 works with Python 2.7 and Python 3.4+ on Windows, macOS, and Linux. It is pure Python code with
 the only 3rd-party dependencies being on [six](https://pypi.python.org/pypi/six),
-[pyparsing](http://pyparsing.wikispaces.com), and [pyperclip](https://github.com/asweigart/pyperclip) 
-(on Windows, [pyreadline](https://pypi.python.org/pypi/pyreadline) is an additional dependency).
+[pyparsing](http://pyparsing.wikispaces.com), and [pyperclip](https://github.com/asweigart/pyperclip).
+Windows has an additional dependency on [pyreadline](https://pypi.python.org/pypi/pyreadline) and Python
+3.4 and earlier have an additional dependency on [contextlib2](https://pypi.python.org/pypi/contextlib2).
 
 For information on other installation options, see
 [Installation Instructions](https://cmd2.readthedocs.io/en/latest/install.html) in the cmd2

docs/install.rst

@@ -128,6 +128,11 @@ If you wish to permanently uninstall ``cmd2``, this can also easily be done with
 
     pip uninstall cmd2
 
+Extra requirement for Python 3.4 and earlier
+--------------------------------------------
+``cmd2`` requires the ``contextlib2`` module for Python 3.4 and earlier.  This is used to temporarily redirect
+stdout and stderr.
+
 Extra requirement for Python 2.7 only
 -------------------------------------
 If you want to be able to pipe the output of commands to a shell command on Python 2.7, then you will need one