Fix _is_builtin_module (#567)

dill/_dill.py

@@ -1588,10 +1588,18 @@ def _is_builtin_module(module):
     # If a module file name starts with prefix, it should be a builtin
     # module, so should always be pickled as a reference.
     names = ["base_prefix", "base_exec_prefix", "exec_prefix", "prefix", "real_prefix"]
-    return any(os.path.realpath(module.__file__).startswith(os.path.realpath(getattr(sys, name)))
-               for name in names if hasattr(sys, name)) or \
-            module.__file__.endswith(EXTENSION_SUFFIXES) or \
-            'site-packages' in module.__file__
+    rp = os.path.realpath
+    # See https://github.com/uqfoundation/dill/issues/566
+    return (
+        any(
+            module.__file__.startswith(getattr(sys, name))
+            or rp(module.__file__).startswith(rp(getattr(sys, name)))
+            for name in names
+            if hasattr(sys, name)
+        )
+        or module.__file__.endswith(EXTENSION_SUFFIXES)
+        or 'site-packages' in module.__file__
+    )
 
 def _is_imported_module(module):
     return getattr(module, '__loader__', None) is not None or module in sys.modules.values()