Add an extra colon to the "data: <file>:<line>" output from failing tests. (#1962)

@gnprice: With this small change my Emacs "compile" mode lets me jump
to that line directly.

Author: gvanrossum

mypy/test/data.py

@@ -405,7 +405,7 @@ def repr_failure(self, excinfo):
             self.parent._prunetraceback(excinfo)
             excrepr = excinfo.getrepr(style='short')
 
-        return "data: {}:{}\n{}".format(self.obj.file, self.obj.line, excrepr)
+        return "data: {}:{}:\n{}".format(self.obj.file, self.obj.line, excrepr)
 
 
 class DataSuite: