fix sysimage argument to julia.core.Julia.__init__ (#482)

On `master` branch, it should be possible to pass a custom "sysimage" to
the `julia.core.Julia.__init__` method. However, if the python version
in use is statically linked, an UnsupportedPythonError is thrown
(despite the sysimage argument having been passed to `__init__`).

This commit prevents the UnsupportedPythonError from being thrown if a
sysimage keyword argument is passed to `__init__`.
Closes #421

src/julia/core.py

@@ -478,8 +478,14 @@ def __init__(self, init_julia=True, jl_init_path=None, runtime=None,
 
             is_compatible_python = jlinfo.is_compatible_python()
             logger.debug("is_compatible_python = %r", is_compatible_python)
+            use_custom_sysimage = options.sysimage is not None
+            logger.debug("use_custom_sysimage = %r", use_custom_sysimage)
             logger.debug("compiled_modules = %r", options.compiled_modules)
-            if not (options.compiled_modules == "no" or is_compatible_python):
+            if not (
+                options.compiled_modules == "no"
+                or is_compatible_python
+                or use_custom_sysimage
+            ):
                 raise UnsupportedPythonError(jlinfo)
 
             self.api.init_julia(options)