fix linter errors in etcdb/sqlparser/sql_tree.py

* Etcd Database Driver version: 1.0.3

### Description

pylint fails on `etcdb/sqlparser/sql_tree.py`

### What I Did

```
$ pylint etcdb/sqlparser/sql_tree.py
************* Module etcdb.sqlparser.sql_tree
C: 1, 0: Missing module docstring (missing-docstring)
C: 1, 0: Missing class docstring (missing-docstring)
R: 1, 0: Too many instance attributes (11/7) (too-many-instance-attributes)
C: 18, 4: Missing method docstring (missing-docstring)
R: 1, 0: Too few public methods (1/2) (too-few-public-methods)

Report
======
15 statements analysed.

Statistics by type
------------------

+---------+-------+-----------+-----------+------------+---------+
|type |number |old number |difference |%documented |%badname |
+=========+=======+===========+===========+============+=========+
|module |1 |NC |NC |0.00 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|class |1 |NC |NC |0.00 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|method |2 |NC |NC |50.00 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|function |0 |NC |NC |0 |0 |
+---------+-------+-----------+-----------+------------+---------+

Raw metrics
-----------

+----------+-------+------+---------+-----------+
|type |number |% |previous |difference |
+==========+=======+======+=========+===========+
|code |17 |85.00 |NC |NC |
+----------+-------+------+---------+-----------+
|docstring |2 |10.00 |NC |NC |
+----------+-------+------+---------+-----------+
|comment |0 |0.00 |NC |NC |
+----------+-------+------+---------+-----------+
|empty |1 |5.00 |NC |NC |
+----------+-------+------+---------+-----------+

Duplication
-----------

+-------------------------+------+---------+-----------+
| |now |previous |difference |
+=========================+======+=========+===========+
|nb duplicated lines |0 |NC |NC |
+-------------------------+------+---------+-----------+
|percent duplicated lines |0.000 |NC |NC |
+-------------------------+------+---------+-----------+

Messages by category
--------------------

+-----------+-------+---------+-----------+
|type |number |previous |difference |
+===========+=======+=========+===========+
|convention |3 |NC |NC |
+-----------+-------+---------+-----------+
|refactor |2 |NC |NC |
+-----------+-------+---------+-----------+
|warning |0 |NC |NC |
+-----------+-------+---------+-----------+
|error |0 |NC |NC |
+-----------+-------+---------+-----------+

Messages
--------

+-----------------------------+------------+
|message id |occurrences |
+=============================+============+
|missing-docstring |3 |
+-----------------------------+------------+
|too-many-instance-attributes |1 |
+-----------------------------+------------+
|too-few-public-methods |1 |
+-----------------------------+------------+

Global evaluation
-----------------
Your code has been rated at 6.67/10
```