lib.wiring: fix Component.signature on subclasses without annotations.

On Python <3.10, classes without annotations do not get an
`__annotations__` member at all, so the `getattr` on a subclass falls
back to the parent class `__annotations__`, attempting to create
signature members twice.  Fix that by looking at the `__dict__` instead.

tests/test_lib_wiring.py

@@ -887,3 +887,16 @@ class ComponentTestCase(unittest.TestCase):
                 r"'rand: In\(Signature\({}\)\.flip\(\)\)' or "
                 r"'rand: Out\(Signature\({}\)\.flip\(\)\)'\?$"):
             PageBuffer()
+
+    def test_inherit(self):
+        class A(Component):
+            clk: In(1)
+
+        class B(A):
+            rst: In(1)
+
+        class C(B):
+            pass
+
+        c = C()
+        self.assertEqual(c.signature, Signature({"clk": In(1), "rst": In(1)}))