Snippet syntax doesn't accept " as a trigger

I have a python snippet for multi-line strings:

snippet " "triple-quoted string"
"""
${1:doc}
"""
endsnippet

The syntax colours " " as a string (snipString) and triple-quoted string" (including that trailing quote) as the trigger (snipStart). The snippet works as expected.

I'm using e4274950abd73acbd48f5acf381f0460f17c6dc9.

I don't know vim syntax files very well, but I think you could create a new syntax group for docstrings that must be at end-of-line. I've attached a patch that does that. It works in my use case and appears to be correct elsewhere. snipString is also used in global, but I don't understand why (and whether that should be a snipDocString too).