[1.8.x] Fixed flake8 warnings on Python 3.

Backport of 47fcbe506c from master

django/db/backends/oracle/operations.py

@@ -235,7 +235,7 @@ WHEN (new.%(col_name)s IS NULL)
         # http://cx-oracle.sourceforge.net/html/cursor.html#Cursor.statement
         # The DB API definition does not define this attribute.
         statement = cursor.statement
-        if statement and six.PY2 and not isinstance(statement, unicode):
+        if statement and six.PY2 and not isinstance(statement, unicode):  # NOQA: unicode undefined on PY3
             statement = statement.decode('utf-8')
         # Unlike Psycopg's `query` and MySQLdb`'s `_last_executed`, CxOracle's
         # `statement` doesn't contain the query parameters. refs #20010.

django/db/utils.py

@@ -19,7 +19,7 @@ DEFAULT_DB_ALIAS = 'default'
 DJANGO_VERSION_PICKLE_KEY = '_django_version'
 
 
-class Error(Exception if six.PY3 else StandardError):
+class Error(Exception if six.PY3 else StandardError):  # NOQA: StandardError undefined on PY3
     pass
 
 

django/utils/functional.py

@@ -310,7 +310,7 @@ class LazyObject(object):
         __bool__ = new_method_proxy(bool)
     else:
         __str__ = new_method_proxy(str)
-        __unicode__ = new_method_proxy(unicode)
+        __unicode__ = new_method_proxy(unicode)  # NOQA: unicode undefined on PY3
         __nonzero__ = new_method_proxy(bool)
 
     # Introspection support

django/utils/html.py

@@ -388,7 +388,7 @@ def html_safe(klass):
             )
         klass_unicode = klass.__unicode__
         klass.__unicode__ = lambda self: mark_safe(klass_unicode(self))
-        klass.__html__ = lambda self: unicode(self)
+        klass.__html__ = lambda self: unicode(self)  # NOQA: unicode undefined on PY3
     else:
         if '__str__' not in klass.__dict__:
             raise ValueError(