[python] Close test API clients

[changhc]

Fixes #19998.

Note that even if the synchronous test API client does not raise any warning when it's not closed, I still updated the template for it so that the behaviour aligns with the async version.

As of the samples, in this PR I updated the asyncio samples only. I did try to update the regular python samples as well, but it seems that most samples are severely outdated, especially for tests because the generator does not overwrite test files if they already exist, though I don't know why there is this rule in the generator, and therefore I would have to update a lot of irrelevant lines along with the core changes in this PR, which does not seem to be a good idea. I'm happy to update the samples in a separate PR.

Test

Generated some code locally:

$ ./mvnw clean install -DskipTests -Dmaven.javadoc.skip=true
$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml -g python -p library=asyncio -o /tmp/out/python
$ pip install -r /tmp/out/python/requirements.txt -r /tmp/out/test-requirements.txt
$ pytest /tmp/out/python
================================================================ test session starts =================================================================
platform linux -- Python 3.8.10, pytest-8.3.4, pluggy-1.5.0
Using --randomly-seed=2004499344
rootdir: /tmp/out/python
configfile: pyproject.toml
plugins: randomly-3.15.0, cov-5.0.0
collected 26 items                                                                                                                                   

/tmp/out/python/test/test_order.py .                                                                                                [  3%]
/tmp/out/python/test/test_user.py .                                                                                                 [  7%]
/tmp/out/python/test/test_api_response.py .                                                                                         [ 11%]
/tmp/out/python/test/test_tag.py .                                                                                                  [ 15%]
/tmp/out/python/test/test_category.py .                                                                                             [ 19%]
/tmp/out/python/test/test_pet_api.py ........                                                                                       [ 50%]
/tmp/out/python/test/test_store_api.py ....                                                                                         [ 65%]
/tmp/out/python/test/test_user_api.py ........                                                                                      [ 96%]
/tmp/out/python/test/test_pet.py .                                                                                                  [100%]

================================================================= 26 passed in 0.54s =================================================================

without the warning mentioned in #19998.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@tomplus Please help review this PR. Thank you!

[wing328]

@changhc thanks for the PR

cc @cbornet (2017/09) @tomplus (2018/10) @krjakbrjak (2023/02) @fa0311 (2023/10) @multani (2023/10)

[multani]

Lgtm
@wing328 any details about the question regarding updating the test samples?

[wing328]

thanks for the PR

i'll take care of the sample update (test files) this weekend