Replace deprecated os.popen() call with Python's built-in file handling

[Edu92337]

Description

This PR refactors the CPU serial reading logic in tools_views.py to use Python's native file I/O instead of the deprecated os.popen() function.

What changed:

• Replaced os.popen("cat /proc/cpuinfo | grep Serial") with direct file reading using open()
• Removed dependency on external shell commands (cat, grep)
• Removed unused import os
• Maintained identical behavior and error handling

Why this change:

• os.popen() has been deprecated since Python 2.6
• Eliminates unnecessary shell spawning, improving security posture for entropy generation
• Removes dependency on external binaries that may not exist in all environments
• Direct file I/O is more performant (avoids spawning shell and piped processes)
• Follows modern Python best practices for reading procfs files

Security context:
Since this code is part of the entropy generation pathway for seed creation in a Bitcoin hardware wallet, eliminating any shell interaction—even with static strings—reduces the attack surface and follows defense-in-depth principles.

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

Checklist

• I've run pytest and made sure all unit tests pass before submitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• No, I'm a fool
• Yes
• N/A (refactor maintains identical behavior, existing tests should cover this)

I have tested this PR on the following platforms/os:

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Other: Local development environment with /proc/cpuinfo verification

Testing methodology:

• Verified Python syntax compiles without errors
• Code review confirms identical logic for extracting Serial field
• Fallback behavior (hash_bytes = b'0') preserved on exceptions

Note: No existing unit tests cover this specific code path (image entropy generation). The refactor maintains identical behavior, so existing integration should work unchanged. I don't currently have access to physical Raspberry Pi hardware, but happy to work with maintainers who have hardware access to validate before merging.

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Fixes #856