Fix flake8 tox.ini directive

Do not specify posargs on flake8 line in pep8 section, as it
breaks using 'tox -e pep8 HEAD~1' to check just code changed
in current change. While here change to match octavia tree
regarding enabled extensions.

Also fixed basepython warning now generated after removing
python2 code.

Trivialfix

Change-Id: Ib4326a1947845a229418b6b4213efc9f2bb12b32

tox.ini

@@ -2,6 +2,7 @@
 minversion = 2.5.0
 envlist = docs,py37,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
@@ -21,7 +22,7 @@ deps =
 commands = stestr run {posargs}
 
 [testenv:pep8]
-commands = flake8 {posargs}
+commands = flake8
            doc8 --ignore-path doc/source/contributor/modules \
            doc/source octavia_lib HACKING.rst README.rst
            # Run security linter
@@ -68,12 +69,16 @@ commands =
 commands = oslo_debug_helper {posargs}
 
 [flake8]
-# E123, E125 skipped as they are invalid PEP-8.
-
+ignore =
 show-source = True
-ignore = E123,E125
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
+# [H106]: Don't put vim configuration in source files
+# [H203]: Use assertIs(Not)None to check for None
+# [H204]: Use assert(Not)Equal to check for equality
+# [H205]: Use assert(Greater|Less)(Equal) for comparison
+# [H904]: Delay string interpolations at logging calls
+enable-extensions=H106,H203,H204,H205,H904
 
 [hacking]
 import_exceptions = octavia_lib.i18n