Fix invalid escape sequence warning.

Python 3.7 warns about unrecognized escape sequences in strings.

Remove the backslash escapes for the `/` character. This doesn't
need to be escaped for re match patterns; it only has special
meaning in sed-style substitution patterns.

Another possible fix is to use raw strings, r"^(https?:)?\/\/"
which leave the backslash characters to the re package to
interpret. It doesn't mind the sequences, but they're still
unnecessary.

flask_nemo/query/resolve.py

@@ -62,7 +62,7 @@ def read(self, uri):
 class HTTPRetriever(RetrieverPrototype):
     """ Http retriever retrieves resources being remotely hosted in CTS
     """
-    __reg_exp__ = re.compile("^(https?:)?\/\/")
+    __reg_exp__ = re.compile("^(https?:)?//")
 
     def match(self, uri):
         """ Check to see if this URI is retrievable by this Retriever implementation