Validate checkpoint callback identity

[3em0]

Proposed change

Validate checkpoint callback identity before restoring request callbacks. Checkpointed requests now store callback name, qualname, module, owner class, and owner module, and restore rejects missing or mismatched callback identities instead of silently binding a request to another spider's same-named method.

This prevents checkpoint resume from dispatching a preserved request/response to the wrong spider callback when different spiders share callback names.

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
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Additional information

• This PR fixes or closes an issue: Fixes Checkpoint callback restoration can bind requests to the wrong spider callback #307
• This PR is related to an issue: Checkpoint callback restoration can bind requests to the wrong spider callback #307
• Link to documentation pull request: **

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doc-strings.

Testing

• python -m pytest tests/spiders/test_request.py tests/spiders/test_engine.py::TestCheckpointMethods tests/spiders/test_templates.py::TestCrawlSpiderPickle tests/spiders/test_sitemap.py::TestSitemapSpiderPickle -q
• python -m pytest tests/spiders -q