Pyparsing incompatibility with peppercompiler in python 2.7

@policorrea identified an error in which the most recent versions of pyparsing are incompatible with Python 2.7 and peppercompiler. The error message for Python 2.7.15rc1 and pyparsing 2.2.0 is:

```
    self.saveAsList = expr.saveAsList
AttributeError: 'str' object has no attribute 'saveAsList'
```

The solution approach will be to require a legacy pyparsing version under python 2.7. 