[python] Some cleanup of samples folder (#17127)

* Delete sample folders of discontinued clients

* Remove duplicate python-flask server sample

The python-flask sample actually lives in samples/server/petstore/python-flask.

* Move hand-written test to "tests" folder

Now, "test" only contains generated stubs and all hand-written tests are in "tests".

* Delete left-over files in Python samples

These are not created by the generators (anymore) and not hand-written
for testing.

* Regenerate test file to fix import error

samples/openapi3/client/petstore/python/test/test_configuration.py

@@ -1,54 +0,0 @@
-# coding: utf-8
-
-"""
-    OpenAPI Petstore
-
-    This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
-
-    OpenAPI spec version: 1.0.0
-    Generated by: https://openapi-generator.tech
-"""
-
-
-from __future__ import absolute_import
-
-import unittest
-
-import petstore_api
-from petstore_api import Configuration # noqa: E501
-from petstore_api.rest import ApiException
-
-
-class TestConfiguration(unittest.TestCase):
-    """Configuration unit test stubs"""
-
-    def setUp(self):
-        self.config= Configuration()
-
-    def tearDown(self):
-        pass
-
-    def test_configuration(self):
-        """Test configuration
-
-        Test host settings # noqa: E501
-        """
-        host_settings = self.config.get_host_settings()
-
-        self.assertEqual('http://{server}.swagger.io:{port}/v2', host_settings[0]['url'])
-        self.assertEqual('petstore', host_settings[0]['variables']['server']['default_value'])
-
-        self.assertEqual('https://localhost:8080/{version}', host_settings[1]['url'])
-        self.assertEqual('v2', host_settings[1]['variables']['version']['default_value'])
-
-    def test_get_host_from_settings(self):
-        """ Test get_host_from_settings
-
-        Test get URL from host settings
-        """
-        self.assertEqual("http://petstore.swagger.io:80/v2", self.config.get_host_from_settings(0))
-        self.assertEqual("http://petstore.swagger.io:8080/v2", self.config.get_host_from_settings(0, {'port': '8080'}))
-        self.assertEqual("http://dev-petstore.swagger.io:8080/v2", self.config.get_host_from_settings(0, {'server': 'dev-petstore', 'port': '8080'}))
-
-if __name__ == '__main__':
-    unittest.main()