[python-flask] adopt python3.5+ syntax (#16375)

* adopt python3.5+ syntax

removing some residual python2 code, since it is not supported anymore, like:
- no need for `six` anymore
- no need for encoding utf8 in top file
- remove `object` inheritance in base model
- remove absolute import `__future__`

* generate samples

applying the new templates

* update python ignore pattern

ignore all the `.venv` folders

samples/server/petstore/python-flask/openapi_server/encoder.py

@@ -1,5 +1,4 @@
 from connexion.apps.flask_app import FlaskJSONEncoder
-import six
 
 from openapi_server.models.base_model import Model
 
@@ -10,7 +9,7 @@ class JSONEncoder(FlaskJSONEncoder):
     def default(self, o):
         if isinstance(o, Model):
             dikt = {}
-            for attr, _ in six.iteritems(o.openapi_types):
+            for attr in o.openapi_types:
                 value = getattr(o, attr)
                 if value is None and not self.include_nulls:
                     continue