Consecutive $$ interpolation fails

A MWE:

```
julia> py"$$('1')$$('2')$$('3')"
3
```

Looks like only the final interpolation is accounted for. Separating the interpolations with spaces makes them work (which in this case leads to a Python Syntax error). I've checked Julia 1.2.0 / PyCall v1.91.1 and Julia 1.3.0-rc3.0 / v1.91.2.

Start by reproducing the given consecutive-$$ MWE with the Julia and PyCall versions listed, then trace the interpolation handling it exercises. Done means each adjacent interpolation is preserved and a regression test covers the example without requiring separating spaces.

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