Write a function that removes every lone 9 that is inbetween 7s.
```python
seven_ate9('79712312') => '7712312'
seven_ate9('79797') => '777'
```
Input: String
Output: String
def seven_ate9(str_):
    x = str_.replace('797','77')
    while str_ != x:
        str_ = x
        x = str_.replace('797','77')
    return str_
def seven_ate9(str_):
    x = str_.replace('797','77')
    while str_ != x:
        str_ = x
        x = str_.replace('797','77')
    return str_