refactor: modernize panda.py to Python 3 with bug fixes, type hints, and CLI

[Blizzard39]

Summary

This PR modernizes panda.py from Python 2 to Python 3, fixes several bugs,
and improves overall code quality and maintainability.

Bug Fixes

• get_driver_types() — self.driver_name in "fglrx" was checking if
  the driver name is a substring of the literal string "fglrx" (e.g.
  "fx" in "fglrx" is True). Fixed with a proper dict-key lookup.
• update_grub_entries() — configured = line != new_line was
  overwriting the flag on every kernel line instead of accumulating it.
  The final value only reflected the last matched line. Fixed with |=.
• Unclosed file handles — multiple open(...).read() calls throughout
  the codebase leaked file descriptors. All converted to with blocks /
  Path.read_text().
• filter() in Python 3 returns a lazy iterator, not a list. Converted
  to a set comprehension to preserve correct behaviour.

Python 3 Modernisation

• All print statements → print() functions (then replaced by logging)
• os.path → pathlib.Path throughout
• %-string formatting → f-strings
• pisi is imported lazily only when actually needed (avoids import errors
  when pisi is not installed)

New Features

• Type hints on all public methods and most private helpers
• logging replaces bare print statements; --debug flag exposes
  DEBUG-level output
• Proper argparse CLI replaces the ad-hoc __main__ test block:
  panda --state, panda --set vendor|os|generic, panda --packages, etc.
• Module-level constants (SYSDIR, DRIVERS_DB, OS_DRIVER_MAP, …) make
  the code easy to test and mock
• GRUB_NEW temp file is cleaned up after writing

Testing

Manually tested against a mock filesystem layout mirroring /sys/bus/pci,
/etc/kernel/, and /boot/grub/grub.conf on Python 3.11.

Notes

• The public API is backwards-compatible except for the rename of
  get_grub_state() → get_driver_state() and
  update_grub_entries() → set_driver_state(), which better describe
  what the methods actually do.
• setup.py can optionally be replaced with pyproject.toml (PEP 517);
  happy to include that as a separate commit if desired.