NO-JIRA: Migrate from github3.py to PyGithub

[RadekManak]

Replace github3.py library with PyGithub for GitHub API interactions. PyGithub has better support for cross-repository PRs and is more actively maintained.

Changes:

• Update dependency: github3.py → PyGithub>=2.8.0
• Migrate all API calls to PyGithub equivalents
• Rewrite authentication using PyGithub Auth classes
• Update test mocks to match PyGithub object structure
• Add null safety check for deleted PR head repositories
• Add missing requests import
• Disable GPG signing in tests

🤖 Generated with Claude Code

[openshift-ci-robot]

@RadekManak: This pull request explicitly references no jira issue.

Details

In response to this:

Replace github3.py library with PyGithub for GitHub API interactions. PyGithub has better support for cross-repository PRs and is more actively maintained.

Changes:

• Update dependency: github3.py → PyGithub>=2.8.0
• Migrate all API calls to PyGithub equivalents
• Rewrite authentication using PyGithub Auth classes
• Update test mocks to match PyGithub object structure
• Add null safety check for deleted PR head repositories
• Add missing requests import
• Disable GPG signing in tests

🤖 Generated with Claude Code

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[openshift-ci]

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[openshift-ci]

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This pull-request has been approved by: RadekManak

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