Use PyDLL(None) to get the handle for the main program

It seems passing None is the way to pass NULL to dlopen.

In master:
https://github.com/python/cpython/blob/4ae8ece5cd4c5853b625381db13429f25512108d/Modules/_ctypes/callproc.c#L1322-L1336

In 2.7 branch:
https://github.com/python/cpython/blob/69e96910153219b0b15a18323b917bd74336d229/Modules/_ctypes/callproc.c#L1431-L1434

julia/core.py

@@ -545,7 +545,7 @@ def __init__(self, init_julia=True, jl_init_path=None, runtime=None,
         else:
             # we're assuming here we're fully inside a running Julia process,
             # so we're fishing for symbols in our own process table
-            self.api = ctypes.PyDLL('')
+            self.api = ctypes.PyDLL(None)
 
         # Store the running interpreter reference so we can start using it via self.call
         self.api.jl_.argtypes = [void_p]