fix: async client always use HTTP_PROXY/HTTPS_PROXY environment variables

[bednar]

Closes #563

Proposed Changes

The underlying async http client (https://docs.aiohttp.org/en/stable/client_reference.html#aiohttp.ClientSession) doesn't always use HTTP_PROXY/HTTPS_PROXY environment variables.

If you want to rely on `HTTP_PROXY/HTTPS_PROXY' env vars, you need to initialize the client by:

async with InfluxDBClientAsync(url="http://localhost:8086", token="my-token", org="my-org", 
                               client_session_kwargs={'trust_env': True}) as client:
     pass

---

Fixed CI build - self signed certificate for testing didn't have correctly set hostname:

https://app.circleci.com/pipelines/github/influxdata/influxdb-client-python/2460/workflows/dfc82ea8-1821-4f1a-a63b-0a4c4c54b832/jobs/13290/tests#failed-test-0

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)