fix PyCall.__init__ when no __main__ present

[marius311]

In some environments, __main__ isn't a module, eg on an ipyparallel worker __main__ is a IPython.core.interactiveshell.DummyMod, causing PyCall to error on loading because that pyeval_ is trying to use __main__ by default. Or worse, if you've compiled a custom sysimage with PyCall (eg for pyjulia), then an error during __init__ means a segfault. This fixes that.

Actually, with this PR, I can now run an ipyparallel cluster of Python workers calling Julia via pyjulia w/ custom sysimage 🎉

$ ipcluster start -n 1 & # start workers
$ ipython
In [1]: import ipyparallel as ipp 
   ...: c = ipp.Client() 
   ...: c.ids                                                                                                                       
Out[1]: [0]

In [2]: %autopx                                                                                                                     
%autopx enabled

In [3]: from julia.api import LibJulia 
   ...: api = LibJulia.load()
   ...: api.sysimage = "sys.so"
   ...: api.init_julia()
   ...: import julia.Main                                                                                                  

In [4]: julia.Main.eval("1+2")                                                                                                      
Out[0:2]: 3

Seems a decent bit of people on the pyjulia Issues have been trying do similar stuff (also with multiprocessing and/or Dask), maybe this will be a helpful solution to point out there assuming this PR makes sense to you guys.

[stevengj]

Looks good, thanks.