`str.join` accepts non-strings

```python
x = 1
y = 2
print(" + ".join([x, y]))
```

Should error out but doesn't:

```console
$ python asd.py     
Traceback (most recent call last):
  File "/Users/tusharsadhwani/code/interpreted/asd.py", line 3, in <module>
    print(" + ".join([x, y]))
          ^^^^^^^^^^^^^^^^^^
TypeError: sequence item 0: expected str instance, int found

$ interpreted asd.py
1 + 2
```