Eliminate six dependency

Less dependencies we have, less effort we need to keep RPM / Deb package
dependencies and building intructions for various OSes up to date.

It was mostly removed in [1]: this commit removed the last occurence and
drops the dependency.

[1]: #263

Author: Totktonada

lib/server_mixins.py

@@ -1,7 +1,13 @@
 import os
 import glob
 import shlex
-from six.moves import shlex_quote
+
+try:
+    # Python 3.3+.
+    from shlex import quote as shlex_quote
+except ImportError:
+    # Python 2.7.
+    from pipes import quote as shlex_quote
 
 from lib.utils import find_in_path
 from lib.utils import print_tail_n

requirements.txt

@@ -2,4 +2,3 @@ PyYAML==5.1
 argparse==1.1
 msgpack-python==0.4.6
 gevent==21.1.2
-six>=1.8.0