I think I found it. After doing the bisect manually (as redirecting stdout/stderr) seems to affect somehow the test) I found commit b0a7a037b8fde56b62f886d5188bced7776777b4 as the culprit. Reverting this commit on the current master seem to solve the problem. This is the diff of the revert:

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 40d7d8af6e..f7d792232e 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -613,52 +613,6 @@ new_dict_with_shared_keys(PyDictKeysObject *keys)
     return new_dict(keys, values);
 }

-
-static PyObject *
-clone_combined_dict(PyDictObject *orig)
-{
-    assert(PyDict_CheckExact(orig));
-    assert(orig->ma_values == NULL);
-    assert(orig->ma_keys->dk_refcnt == 1);
-
-    Py_ssize_t keys_size = _PyDict_KeysSize(orig->ma_keys);
-    PyDictKeysObject *keys = PyObject_Malloc(keys_size);
-    if (keys == NULL) {
-        PyErr_NoMemory();
-        return NULL;
-    }
-
-    memcpy(keys, orig->ma_keys, keys_size);
-
-    /* After copying key/value pairs, we need to incref all
-       keys and values and they are about to be co-owned by a
-       new dict object. */
-    PyDictKeyEntry *ep0 = DK_ENTRIES(keys);
-    Py_ssize_t n = keys->dk_nentries;
-    for (Py_ssize_t i = 0; i < n; i++) {
-        PyDictKeyEntry *entry = &ep0[i];
-        PyObject *value = entry->me_value;
-        if (value != NULL) {
-            Py_INCREF(value);
-            Py_INCREF(entry->me_key);
-        }
-    }
-
-    PyDictObject *new = (PyDictObject *)new_dict(keys, NULL);
-    if (new == NULL) {
-        /* In case of an error, `new_dict()` takes care of
-           cleaning up `keys`. */
-        return NULL;
-    }
-    new->ma_used = orig->ma_used;
-    assert(_PyDict_CheckConsistency(new));
-    if (_PyObject_GC_IS_TRACKED(orig)) {
-        /* Maintain tracking. */
-        _PyObject_GC_TRACK(new);
-    }
-    return (PyObject *)new;
-}
-
 PyObject *
 PyDict_New(void)
 {
@@ -2527,13 +2481,7 @@ PyDict_Copy(PyObject *o)
         PyErr_BadInternalCall();
         return NULL;
     }
-
     mp = (PyDictObject *)o;
-    if (mp->ma_used == 0) {
-        /* The dict is empty; just return a new dict. */
-        return PyDict_New();
-    }
-
     if (_PyDict_HasSplitTable(mp)) {
         PyDictObject *split_copy;
         Py_ssize_t size = USABLE_FRACTION(DK_SIZE(mp->ma_keys));
@@ -2560,27 +2508,6 @@ PyDict_Copy(PyObject *o)
             _PyObject_GC_TRACK(split_copy);
         return (PyObject *)split_copy;
     }
-
-    if (PyDict_CheckExact(mp) && mp->ma_values == NULL &&
-            (mp->ma_used >= (mp->ma_keys->dk_nentries * 2) / 3))
-    {
-        /* Use fast-copy if:
-
-           (1) 'mp' is an instance of a subclassed dict; and
-
-           (2) 'mp' is not a split-dict; and
-
-           (3) if 'mp' is non-compact ('del' operation does not resize dicts),
-               do fast-copy only if it has at most 1/3 non-used keys.
-
-           The last condition (3) is important to guard against a pathological
-           case when a large dict is almost emptied with multiple del/pop
-           operations and copied after that.  In cases like this, we defer to
-           PyDict_Merge, which produces a compacted copy.
-        */
-        return clone_combined_dict(mp);
-    }
-
     copy = PyDict_New();
     if (copy == NULL)
         return NULL;