pyosys: __getitem__ for supported classes

- functions that have a const `[]` operator method now support `__getitem__` in Python
- fields of a pointer type now return a `reference_internal` instead of a `copy` because classes referenced to by pointers typically aren't copyable (e.g. RTLIL::Wire, RTLIL::Module, etc)
- removed duplicate of test_script.py

tests/pyosys/test_sigspec_it.py

@@ -0,0 +1,28 @@
+from pyosys import libyosys as ys
+from pathlib import Path
+
+__file_dir__ = Path(__file__).absolute().parent
+
+def _dump_sigbit(bit):
+    if bit.is_wire():
+        if bit.wire.width == 1:
+            return bit.wire.name.str()
+        else:
+            return f"{bit.wire.name} [{bit.offset}]"
+    else:
+        if bit.data == ys.State.S1:
+            return 1
+        elif bit.data == ys.State.S0:
+            return 0
+        else:
+            assert "unknown constants not supported"
+
+d = ys.Design()
+
+ys.run_pass(f"read_verilog {__file_dir__ / 'spm.cut.v.gz'}", d)
+ys.run_pass(f"hierarchy -top spm", d)
+module = d.module(r"\spm")
+for conn_from, conn_to in module.connections_:
+    for bit_from, bit_to in zip(conn_from, conn_to):
+        print(f"assign {_dump_sigbit(bit_from)} = {_dump_sigbit(bit_to)};")
+