Add python3 to bindep

We recently switched to using python3, so let's add the files to
bindep.txt as well so that the packages get installed if needed.

Note this is a nop in OpenStack CI since python3 is available already.
But this is important to document dependencies.

Change-Id: Ic6c1c9adcec938c0db4f207d1d8e523e96ffe740

bindep.txt

@@ -12,6 +12,10 @@ libjpeg-dev [platform:dpkg]
 libjpeg-turbo-devel [platform:rpm]
 python-dev [platform:dpkg]
 python-devel [platform:rpm]
+python3-all [platform:dpkg !platform:ubuntu-precise]
+python3-all-dev [platform:dpkg !platform:ubuntu-precise]
+python3-devel [platform:fedora]
+python34-devel [platform:centos]
 zlib-devel [platform:rpm]
 zlib1g-dev [platform:dpkg]
 inkscape