Fix pep8 error in py37

While running PEP8 in python3.7 environment, will met the
following exception:
"RuntimeError: generator raised StopIteration"
This is an known issue in py3.7, and had been fixed in Pylint 2.x.

After upgrade Pylint to 2.2.0, the issue is:
"AttributeError: 'Import' object has no
attribute 'infer_name_module' on import"
This issue was fixed in Pylint 2.3.0.

So upgrades Pylint to 2.3.0, this patch also
adds some refactor recommendations in .pylintrc[1]

[1] http://paste.openstack.org/show/789057/

Change-Id: I2b3983e3653d92c5b12f233b57fb4467df1da83d

.pylintrc

@@ -72,8 +72,12 @@ disable=
 # "R" Refactor recommendations
  abstract-class-little-used,
  abstract-class-not-used,
+ consider-using-set-comprehension,
  duplicate-code,
+ inconsistent-return-statements,
  interface-not-implemented,
+ no-else-raise,
+ no-else-return,
  no-self-use,
  too-few-public-methods,
  too-many-ancestors,
@@ -84,7 +88,8 @@ disable=
  too-many-locals,
  too-many-public-methods,
  too-many-return-statements,
- too-many-statements
+ too-many-statements,
+ useless-object-inheritance
 
 [BASIC]
 # Variable names can be 1 to 31 characters long, with lowercase and underscores

test-requirements.txt

@@ -7,7 +7,7 @@ coverage!=4.4,>=4.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 flake8-import-order==0.12 # LGPLv3
 mock>=2.0.0 # BSD
-pylint==1.7.6 # GPLv2
+pylint==2.3.0 # GPLv2
 requests-mock>=1.2.0 # Apache-2.0
 sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0