Tune httpx keep alives for polling integrations

[bdraco]

Proposed change

aiohttp closes the connection after 15s by default, and httpx closes the connection after 5s by default.

We have a lot of integrations that poll every 10-30s which create and tear down connections over and over.

Set the keep alive time to 15s for httpx to match aiohttp to maximize connection reuse and avoid tls negotiation overhead

The server side default for nginx is 75s https://nginx.org/en/docs/http/ngx_http_core_module.html#keepalive_timeout which means we likely close a lot of connections that could be reused with the defaults

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[bdraco]

Still testing with this, but I think we can go as low as 31s and still be good for 90% of the cases

[bdraco]

At 32 there is still a nice drop in connection churn and less risk of side effects from non-compliant webservers

[balloob]

Should we follow the 75s too like nginx has ?