Add n_keys option to Dict.empty()

See numba#8110

numba/cext/dictobject.c

@@ -563,18 +563,26 @@ numba_dict_new(NB_Dict** out, Py_ssize_t size, Py_ssize_t key_size, Py_ssize_t v
 /* Allocate a new dictionary with enough space to hold n_keys without resizes */
 int
 numba_dict_new_noresize(NB_Dict** out, Py_ssize_t n_keys, Py_ssize_t key_size, Py_ssize_t val_size) {
+
+    if (n_keys <= USABLE_FRACTION(D_MINSIZE)) {
+        return numba_dict_new(out, D_MINSIZE, key_size, val_size);
+    }
+
     Py_ssize_t size;
 
-    size = n_keys + (n_keys >> 1); // Relies on a load factor of 2/3.
+    size = n_keys + (n_keys >> 1) + 1; // Relies on a load factor of 2/3.
 
     /* Round up to the nearest power of 2.
        Overflows when result is 2^(8*sizeof(Py_ssize_t)-1) due to signed Py_ssize_t.
-       Could pose a real problem on 32-bit systems. */
+       Could be problematic on 32-bit systems. */
+
     size--;
+
     uint32_t shift;
     for (shift = 1; shift < sizeof(Py_ssize_t) * CHAR_BIT; shift <<= 1) {
         size |= (size >> shift);
     }
+
     size++;
 
     return numba_dict_new(out, size, key_size, val_size);