[MRG] Allow the use of "if" in the shorthand generator syntax

[mstimberg]

I noticed this a while ago but never got around fixing it.
At the moment, the short-hand generator syntax (for single targets per source) does not work with if, because something like connect(j='i if i % 2 == 0') is translated into i if i % 2 == 0 for _ in range(1).
My commit is a bit of a quick fix, I think we should probably include the parsing of the short-hand code into the parse_generator_syntax function in the long run to make the code easier to follow.

Either way, I'd be happy if this could be merged as a short-term solution, I have a use case for this and writing for _ in range(1) is really ugly...

[thesamovar]

Seems reasonable to me!