Fix CVE-2018-1061:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-1061

Taken from upstream commit (sans test and NEWS):
https://github.com/python/cpython/commit/e052d40cea15f582b50947f7d906b39744dc62a2

diff --git a/Lib/difflib.py b/Lib/difflib.py
index 1c6fbdbedcb7..788a92df3f89 100644
--- a/Lib/difflib.py
+++ b/Lib/difflib.py
@@ -1103,7 +1103,7 @@ def _qformat(self, aline, bline, atags, btags):
 
 import re
 
-def IS_LINE_JUNK(line, pat=re.compile(r"\s*#?\s*$").match):
+def IS_LINE_JUNK(line, pat=re.compile(r"\s*(?:#\s*)?$").match):
     r"""
     Return 1 for ignorable line: iff `line` is blank or contains a single '#'.