[python,aiohttp] Don't create persistent aiohttp.ClientSession in __init__

[dcermak]

aiohttp's ClientSession & TCPConnector used to obtain an event loop in init (via asyncio.get_event_loop). However, as of aio-libs/aiohttp#8512 both classes now obtain the running event loop and won't potentially create one. This makes it impossible to create ClientSession and TCPConnector objects outside of coroutines, as get_running_loop must be called from a coroutine.

Thus we defer the creation of a ClientSession into the actual request and cache it for later usage. Thereby we pay only a very small price on the first request, but subsequent requests will not be any more expensive.

This PR should fix a long standing issue in py-gitea-opensuse-org: https://github.com/dcermak/py-gitea-opensuse-org/actions/runs/12208693521/job/34062321907?pr=48 which broke with a recent release of aiohttp. I have tested this PR to regenererate py-gitea-opensuse-org and with this PR, the tests pass

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
  

• 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.

cc @cbornet @tomplus @krjakbrjak @fa0311 @multani

[wing328]

given that you've tested it in your use cases to confirm the fix, we can give it a try.

thanks for the PR