Python : Add Xpath injection query

This PR adds support for detecting XPATH injection in Python.
I have included the ql files as well as the tests with this.

python/ql/test/query-tests/Security/lib/lxml/etree/__init__.py

@@ -0,0 +1,37 @@
+class _ElementTree(object):
+    def xpath(self, _path, namespaces=None, extensions=None, smart_strings=True, **_variables):
+        pass
+
+    def xslt(self, _xslt, extensions=None, access_control=None, **_kw):
+        pass
+
+
+class ETXPath(object):
+    def __init__(self, path, extensions=None, regexp=True, smart_strings=True):
+        pass
+
+
+class XPath(object):
+    def __init__(self, path, namespaces=None, extensions=None, regexp=True, smart_strings=True):
+        pass
+
+
+class XSLT(object):
+    def __init__(self, xslt_input, extensions=None, regexp=True, access_control=None):
+        pass
+
+
+def parse(self, parser=None, base_url=None):
+    return _ElementTree()
+
+
+def fromstring(self, text, parser=None, base_url=None):
+    pass
+
+
+def fromstringlist(self, strings, parser=None):
+    pass
+
+
+def XML(self, text, parser=None, base_url=None):
+    pass