Square Brackets Indexing Strings in Parse

JSONPATH Square brackets indicate the handling of JSON arrays.

Currently, brackets are indexing strings. For example, the following doesn't return the expected output None but ‘e’. This is incorrect in JSONPATH
```
import json
from jsonpath_ng.ext import parse

json_data = {‘test_key’: ‘test_value’}

jsonpath_expression = parse('$.test_key[0]')[0].value
```