fix linter errors in etcdb/constants/FIELD_TYPE.py

* Etcd Database Driver version: 1.0.3

### Description

pylint fails on `etcdb/constants/FIELD_TYPE.py`

### What I Did

```
$ pylint etcdb/constants/FIELD_TYPE.py
************* Module etcdb.constants.FIELD_TYPE
C: 1, 0: Invalid module name "FIELD_TYPE" (invalid-name)

Report
======
29 statements analysed.

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

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

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

+----------+-------+------+---------+-----------+
|type |number |% |previous |difference |
+==========+=======+======+=========+===========+
|code |29 |76.32 |NC |NC |
+----------+-------+------+---------+-----------+
|docstring |6 |15.79 |NC |NC |
+----------+-------+------+---------+-----------+
|comment |0 |0.00 |NC |NC |
+----------+-------+------+---------+-----------+
|empty |3 |7.89 |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 |1 |NC |NC |
+-----------+-------+---------+-----------+
|refactor |0 |NC |NC |
+-----------+-------+---------+-----------+
|warning |0 |NC |NC |
+-----------+-------+---------+-----------+
|error |0 |NC |NC |
+-----------+-------+---------+-----------+

Messages
--------

+-------------+------------+
|message id |occurrences |
+=============+============+
|invalid-name |1 |
+-------------+------------+

Global evaluation
-----------------
Your code has been rated at 9.66/10
```