NodeSet: declare RegEx pattern as a raw string (#414)

Python 3 interprets string literals as Unicode strings. \D is treated as
an unrecognized escape sequence and as such is left unchanged.

from 2.4.1. String and Bytes literals:
https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals

Changed in version 3.6: Unrecognized escape sequences produce a
DeprecationWarning. In some future version of Python they will be a
SyntaxError.

Closes #414.

Change-Id: I2f756a4647d740adb0bc35499fe869121993069f

Author: thiell

lib/ClusterShell/NodeSet.py

@@ -790,7 +790,7 @@ def __init__(self, group_resolver, node_wildcard_enable=True):
         Initialize Parsing Engine.
         """
         self.group_resolver = group_resolver
-        self.base_node_re = re.compile("(\D*)(\d*)")
+        self.base_node_re = re.compile(r"(\D*)(\d*)")
         self.node_wc = node_wildcard_enable  # node wildcard support
 
     def parse(self, nsobj, autostep):