Extracted watchdog into separate dependency group

[david-yz-liu]

Proposed Changes

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This PR extracts watchdog into a separate dependency group, so that it only needs to be installed when users want to enable the watch configuration option for python_ta.check_all. watchdog isn't available on pyiodide, which is a direction I'd like us to explore for PythonTA in the future.

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Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)	X
🐛 Bug fix (non-breaking change that fixes an issue)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📚 Documentation update (change that only updates documentation)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

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