Enhance python with snippet for UltiSnips

* the `with` snippet in UltiSnips now contains a VISUAL element
* The `as` keyword portion of a with statement now uses python
  interpolation to determine presence

UltiSnips/python.snippets

@@ -16,6 +16,12 @@ if __name__ == `!p snip.rv = get_quoting_style(snip)`__main__`!p snip.rv = get_q
 	${1:${VISUAL:main()}}
 endsnippet
 
+snippet with "with" b
+with ${1:expr}`!p snip.rv = " as " if t[2] else ""`${2:var}:
+	${3:${VISUAL:pass}}
+${0}
+endsnippet
+
 snippet for "for loop" b
 for ${1:item} in ${2:iterable}:
 	${3:${VISUAL:pass}}