fix linter errors in etcdb/etcdtimestamp.py

* Etcd Database Driver version: 1.0.3

### Description

pylint fails on `etcdb/etcdtimestamp.py`

### What I Did

```
$ pylint etcdb/etcdtimestamp.py
************* Module etcdb.etcdtimestamp
C: 1, 0: Missing module docstring (missing-docstring)
R: 9, 4: Too many arguments (7/5) (too-many-arguments)
C: 14, 4: Missing method docstring (missing-docstring)
C: 18, 4: Missing method docstring (missing-docstring)
C: 22, 4: Missing method docstring (missing-docstring)
C: 26, 4: Missing method docstring (missing-docstring)
C: 30, 4: Missing method docstring (missing-docstring)
C: 34, 4: Missing method docstring (missing-docstring)

Report
======
18 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 |7 |7 |= |14.29 |0.00 |
+---------+-------+-----------+-----------+------------+---------+
|function |0 |0 |= |0 |0 |
+---------+-------+-----------+-----------+------------+---------+

External dependencies
---------------------
::

etcdb
\-etcddate (etcdb.etcdtimestamp)
\-etcdtime (etcdb.etcdtimestamp)

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

+----------+-------+------+---------+-----------+
|type |number |% |previous |difference |
+==========+=======+======+=========+===========+
|code |25 |69.44 |25 |= |
+----------+-------+------+---------+-----------+
|docstring |3 |8.33 |3 |= |
+----------+-------+------+---------+-----------+
|comment |0 |0.00 |0 |= |
+----------+-------+------+---------+-----------+
|empty |8 |22.22 |8 |= |
+----------+-------+------+---------+-----------+

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

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

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

+-----------+-------+---------+-----------+
|type |number |previous |difference |
+===========+=======+=========+===========+
|convention |7 |7 |= |
+-----------+-------+---------+-----------+
|refactor |1 |1 |= |
+-----------+-------+---------+-----------+
|warning |0 |0 |= |
+-----------+-------+---------+-----------+
|error |0 |0 |= |
+-----------+-------+---------+-----------+

Messages
--------

+-------------------+------------+
|message id |occurrences |
+===================+============+
|missing-docstring |7 |
+-------------------+------------+
|too-many-arguments |1 |
+-------------------+------------+

Global evaluation
-----------------
Your code has been rated at 5.56/10 (previous run: 5.56/10, +0.00)
```