Remove pycodestyle version pin. Add E402 and W503 to ignore.

Rather than pinning pycodestyle, ignore the E402 and W503 error. Fix
issue with E731 (Do not assign a lambda expression, use a def).

W503 is something we will likely never enable as it is a personal
style decision and can change depending on the code. There is no one
right answer. Interestingly there is also a W504 which is the opposite
check.

E402 is one we should most likely fix. But it can be done in a follow
patch or patches.

List of various error codes:
https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes

Change-Id: Ie2c31a8a3c75beeef22453cab5878cf0094bdf3f
(cherry picked from commit ca91d4d871)

ironic/tests/unit/common/test_images.py

@@ -261,7 +261,8 @@ class FsImageTestCase(base.TestCase):
     def test__create_root_fs(self, path_exists_mock,
                              dirname_mock, mkdir_mock, cp_mock):
 
-        path_exists_mock_func = lambda path: path == 'root_dir'
+        def path_exists_mock_func(path):
+            return path == 'root_dir'
 
         files_info = {
             'a1': 'b1',

test-requirements.txt

@@ -28,4 +28,3 @@ sphinxcontrib-seqdiag # BSD
 openstackdocstheme>=1.16.0 # Apache-2.0
 reno!=2.3.1,>=1.8.0 # Apache-2.0
 os-api-ref>=1.0.0 # Apache-2.0
-pycodestyle<=2.3.1

tox.ini

@@ -95,7 +95,9 @@ setenv = PYTHONHASHSEED=0
 commands = {posargs}
 
 [flake8]
-ignore = E129
+# [E402] Module level import not at top of file
+# [W503] Line break before binary operator.
+ignore = E129,E402,W503
 exclude =  .venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import-order-style = pep8
 application-import-names = ironic