Apply Python3 modernize transforms

Main changes:
- use of __future__
- using six lib to:
	- deal with checking unicode
	- dict operations
	- other API changes

schema_salad/jsonld_context.py

@@ -7,6 +7,7 @@
 from six.moves import urllib
 
 import ruamel.yaml as yaml
+import six
 try:
     from ruamel.yaml import CSafeLoader as SafeLoader
 except ImportError:

schema_salad/makedoc.py

@@ -18,6 +18,8 @@
 from six import StringIO
 
 from typing import cast, Any, Dict, IO, List, Optional, Set, Text, Union
+import six
+from six.moves import range
 
 _logger = logging.getLogger("salad")
 

schema_salad/ref_resolver.py

@@ -32,6 +32,8 @@
 import xml.sax
 from typing import (cast, Any, AnyStr, Callable, Dict, List, Iterable,
         Optional, Set, Text, Tuple, TypeVar, Union)
+import six
+from six.moves import range
 
 
 _logger = logging.getLogger("salad")

schema_salad/validate.py

@@ -12,6 +12,8 @@
 
 from typing import Any, List, Set, Union, Text
 from .sourceline import SourceLine, lineno_re, bullets, indent
+import six
+from six.moves import range
 
 _logger = logging.getLogger("salad")