Retry Lando requests

[marco-c]

This will help fixmozilla/code-review#3537.

[Copilot]

Pull request overview

This PR introduces a shared, retry-enabled requests.Session helper and migrates Lando/Phabricator HTTP calls to use it, aiming to improve reliability of Lando requests (per the linked issue).

Changes:

• Add utils.get_session() that returns a cached requests.Session configured with urllib3.Retry and a larger connection pool.
• Switch libmozdata/lando.py and libmozdata/phabricator.py HTTP calls from direct requests.* usage to get_session(...).*.
• Add an explicit urllib3 dependency in requirements.txt to support direct Retry usage.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
requirements.txt	Adds urllib3 as an explicit dependency for retry support.
libmozdata/utils.py	Introduces cached, retry-configured requests.Session factory.
libmozdata/phabricator.py	Uses shared retry-enabled session for Conduit POST requests (and removes prior bespoke retry loop).
libmozdata/lando.py	Uses shared retry-enabled session for Lando GET/POST/DELETE calls.

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[suhaibmujahid]

LGTM! The test failure seems unrelated.