text: Title: SyntaxWarning with python 3.8
  Body: Hello,
  a SyntaxWarning occurs when using SAlib with python 3.8
  ```
  \lib\site-packages\SALib\util\_
  _init__.py:222: SyntaxWarning: "is" with a literal. Did you mean "=="?
    elif row['group'] is 'NA':
  \lib\site-packages\SALib\util\r
  esults.py:15: SyntaxWarning: "is not" with a literal. Did you mean "!="?
    return pd.DataFrame({k: v for k, v in self.items() if k is not 'names'},
  ```