Bug: Non-constant value for `sep` does not work inside `print()`

```python
seperator: str = "#"
print(1, 2, 3, 4, 5, sep=seperator)
```
#### CPython
```console
(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ python ./examples/example.py
1#2#3#4#5
```

#### LPython
```console
(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
12345
```