v.db.select: Rewrite JSON output using parson library

[saket0187]

This PR addresses #6969.
(I know there is already a PR, but it seems inactive for one week and not making progress, so I made this PR.)

This PR rewrites v.db.select using the parson wrapper. I have tried to write it in a cleaner and more readable way. Also, I have added one extent test as @wenzeslaus asked for in the other PR. However, I didn't add the CSV one he requested because I think there are enough tests that check format=csv in the test_v_db_select_json_csv.py file. So I haven't added that.

Apart from that, I have changed assertLooksLike to assertEqual for the JSON test because using that was giving an error due to a difference in structure, I think (see below):

If using assertLooksLike is really needed, then we will have to update `out_json`, but I think it's good as it is—clean and compact. Adding a new style will only increase the number of lines in the file.

After this, all the test cases are passing successfully on my side.

Finally, this is how the JSON output looks (just in case you want to see):

{
    "info": {
        "columns": [
            {
                "name": "cat",
                "sql_type": "INTEGER",
                "is_number": true
            },
            {
                "name": "MAJORRDS_",
                "sql_type": "DOUBLE PRECISION",
                "is_number": true
            },
            {
                "name": "ROAD_NAME",
                "sql_type": "CHARACTER",
                "is_number": false
            },
            {
                "name": "MULTILANE",
                "sql_type": "CHARACTER",
                "is_number": false
            },
            {
                "name": "PROPYEAR",
                "sql_type": "INTEGER",
                "is_number": true
            },
            {
                "name": "OBJECTID",
                "sql_type": "INTEGER",
                "is_number": true
            },
            {
                "name": "SHAPE_LEN",
                "sql_type": "DOUBLE PRECISION",
                "is_number": true
            }
        ]
    },
    "records": [
        {
            "cat": 1,
            "MAJORRDS_": 1,
            "ROAD_NAME": "NC-50",
            "MULTILANE": "no",
            "PROPYEAR": 0,
            "OBJECTID": 1,
            "SHAPE_LEN": 4825.3694050000004
        },
        {
            "cat": 2,
            "MAJORRDS_": 2,
            "ROAD_NAME": "NC-50",
            "MULTILANE": "no",
            "PROPYEAR": 0,
            "OBJECTID": 2,
            "SHAPE_LEN": 14392.589058
        },
        {
            "cat": 3,
            "MAJORRDS_": 3,
            "ROAD_NAME": "NC-98",
            "MULTILANE": "no",
            "PROPYEAR": 0,
            "OBJECTID": 3,
            "SHAPE_LEN": 3212.9812419999998
        }
    ]
}

for extent

{
    "extent": {
        "n": 201971.85945864301,
        "s": 148158.10953755351,
        "w": 123971.19498978264,
        "e": 209096.26602160058
    }
}