Avoid importing six. (#112)

Author: felixfontein

changelogs/fragments/six.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "Avoid importing ``ansible.module_utils.six`` (https://github.com/felixfontein/ansible-acme/pull/112)."

plugins/filter/_substitute_dns.py

@@ -56,8 +56,9 @@
   type: string
 '''
 
+import sys
+
 from ansible.errors import AnsibleFilterError
-from ansible.module_utils.six import string_types
 from ansible.module_utils.common.text.converters import to_text, to_native
 
 try:
@@ -66,6 +67,11 @@
     # Python 2.x compat
     from collections import Mapping
 
+if sys.version_info >= (3,):
+    string_types = (str,)
+else:
+    string_types = basestring  # pylint: disable=undefined-variable
+
 
 def substitute_dns(name, substitution_map):
     if not isinstance(name, string_types):