fix linter errors in etcdb/etcdstring.py

* Etcd Database Driver version: 1.0.3

### Description

linter fails on `etcdb/etcdstring.py`
### What I Did

```
$ pylint etcdb/etcdstring.py
************* Module etcdb.etcdstring
C: 1, 0: Missing module docstring (missing-docstring)
R: 1, 0: Too few public methods (0/2) (too-few-public-methods)

Report
======
3 statements analysed.

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

+---------+-------+-----------+-----------+------------+---------+
|type |number |old number |difference |%documented |%badname |
+=========+=======+===========+===========+============+=========+
|module |1 |1 |= |0.00 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|class |1 |1 |= |100.00 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|method |1 |1 |= |100.00 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|function |0 |0 |= |0 |0 |
+---------+-------+-----------+-----------+------------+---------+

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

+----------+-------+------+---------+-----------+
|type |number |% |previous |difference |
+==========+=======+======+=========+===========+
|code |4 |80.00 |4 |= |
+----------+-------+------+---------+-----------+
|docstring |1 |20.00 |1 |= |
+----------+-------+------+---------+-----------+
|comment |0 |0.00 |0 |= |
+----------+-------+------+---------+-----------+
|empty |0 |0.00 |0 |= |
+----------+-------+------+---------+-----------+

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

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

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

+-----------+-------+---------+-----------+
|type |number |previous |difference |
+===========+=======+=========+===========+
|convention |1 |1 |= |
+-----------+-------+---------+-----------+
|refactor |1 |1 |= |
+-----------+-------+---------+-----------+
|warning |0 |0 |= |
+-----------+-------+---------+-----------+
|error |0 |0 |= |
+-----------+-------+---------+-----------+

Messages
--------

+-----------------------+------------+
|message id |occurrences |
+=======================+============+
|too-few-public-methods |1 |
+-----------------------+------------+
|missing-docstring |1 |
+-----------------------+------------+

Global evaluation
-----------------
Your code has been rated at 3.33/10 (previous run: 3.33/10, +0.00)
```