docs: Fix live-preview with python 3.8

Ubuntu 20.04 ships Python 3.8 by default, which causes `make
live-preview` failure during one of the dependency installations in the
pipenv unless we bump the "six" version.

While we're at it, sort alphabetically.

Signed-off-by: Joe Stringer <joe@cilium.io>

Documentation/requirements.txt

@@ -15,7 +15,7 @@ PyYAML==5.3.1
 recommonmark==0.4.0
 requests==2.20.0
 semver==2.9.0
-six==1.11.0
+six==1.15.0
 snowballstemmer==1.2.1
 Sphinx==1.8.1
 sphinx-autobuild==0.7.1