linkcheck builder: ensure error response is removed-from-scope before fallback request

[jayaddison]

Feature or Bugfix

• Bugfix

Purpose

• Ensures that the connection from the initial HTTP HEAD request may be re-used for the HTTP GET in the case of failures.

Detail

• Initially discovered during Flaky unit tests: linkcheck builder (tests/test_build_linkcheck.py) #11299; temporarily worked-around by increased timeouts, but re-appeared during development of linkcheck builder: begin using session-based HTTP requests #11340.
• Tested as part of Testing: pr 11340 test flakiness jayaddison/sphinx#2.

Relates

• Resolves Flaky unit tests: linkcheck builder (tests/test_build_linkcheck.py) #11299.

[jayaddison]

(please hold-off on merging until after the 6.2.0 release)

[jayaddison]

I'm undecided about whether this is a valid fix. It is included in #11340 currently, where linkchecker unit test failures continue to occur. I'll move this into draft status.

[jayaddison]

I don't believe that the change here is required, so I'm going to close this and drop the relevant commits while rebasing #11340.

[jayaddison]

My best-effort attempt to explain why I think this may have been useful is that I'm not convinced that client-side resources (particularly, connections and sockets) are being closed and returned to the urllib3 connection pool.

Although I don't think that an unused variable continuing to exist within the scope of a method should make a semantic difference, I felt that by explicitly removing it from scope using del, it might make garbage-collection and cleanup of the associated object more likely.

I don't feel like I've proven or disproven that, though - and so it seems better not to include the change at all.