[python] Update python versions (#8042)

* Remove redundant encoding definitions

UTF-8 is already the default encoding in Python 3.

* Remove Python3.4 related requirements

* Remove dead Python version 3.5

* Add Python 3.9 to CI and test configs

* Update petstore example

Author: ulgens

modules/openapi-generator/src/main/resources/python/README.mustache

@@ -17,7 +17,7 @@ For more information, please visit [{{{infoUrl}}}]({{{infoUrl}}})
 
 ## Requirements.
 
-Python >= 3.5
+Python >= 3.6
 
 ## Installation & Usage
 ### pip install

modules/openapi-generator/src/main/resources/python/README_onlypackage.mustache

@@ -17,7 +17,7 @@ For more information, please visit [{{{infoUrl}}}]({{{infoUrl}}})
 
 ## Requirements.
 
-Python >= 3.5
+Python >= 3.6
 
 ## Installation & Usage
 

modules/openapi-generator/src/main/resources/python/__init__apis.mustache

@@ -1,7 +1,6 @@
 {{#apiInfo}}
 {{#apis}}
 {{#-first}}
-# coding: utf-8
 
 # flake8: noqa
 

modules/openapi-generator/src/main/resources/python/__init__models.mustache

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 # flake8: noqa
 
 # import all models into this package

modules/openapi-generator/src/main/resources/python/__init__package.mustache

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 # flake8: noqa
 
 {{>partial_header}}

modules/openapi-generator/src/main/resources/python/api.mustache

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 {{>partial_header}}
 
 import re  # noqa: F401

modules/openapi-generator/src/main/resources/python/api_client.mustache

@@ -1,4 +1,3 @@
-# coding: utf-8
 {{>partial_header}}
 
 import json

modules/openapi-generator/src/main/resources/python/api_test.mustache

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 {{>partial_header}}
 
 import unittest

modules/openapi-generator/src/main/resources/python/asyncio/rest.mustache

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 {{>partial_header}}
 
 import io

modules/openapi-generator/src/main/resources/python/configuration.mustache

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 {{>partial_header}}
 
 import copy