[mypyc] Fixing InitVar for dataclasses. (#18319)

Fixes mypyc/mypyc#934.

`InitVar` variables are not attributes of a dataclass `PyTypeObject`.
Adding check before removing `InitVar` keys from `PyTypeObject` in
`CPyDataclass_SleightOfHand`.

mypyc/lib-rt/misc_ops.c

@@ -365,7 +365,8 @@ CPyDataclass_SleightOfHand(PyObject *dataclass_dec, PyObject *tp,
     pos = 0;
     PyObject *key;
     while (PyDict_Next(annotations, &pos, &key, NULL)) {
-        if (PyObject_DelAttr(tp, key) != 0) {
+        // Check and delete key. Key may be absent from tp for InitVar variables.
+        if (PyObject_HasAttr(tp, key) == 1 && PyObject_DelAttr(tp, key) != 0) {
             goto fail;
         }
     }

mypyc/test-data/run-classes.test

@@ -2655,3 +2655,21 @@ import native
 [out]
 (31, 12, 23)
 (61, 42, 53)
+
+[case testDataclassInitVar]
+import dataclasses
+
+@dataclasses.dataclass
+class C:
+    init_v: dataclasses.InitVar[int]
+    v: float = dataclasses.field(init=False)
+
+    def __post_init__(self, init_v):
+        self.v = init_v + 0.1
+
+[file driver.py]
+import native
+print(native.C(22).v)
+
+[out]
+22.1