Add Python 3.13 support and drop Python 3.8 support

[randomir]

• [x] ~Note: VirtualGraphComposite tests are expected to fail on 3.13 until we either upgrade dwave-drivers to support numpy2 (required by py313), or we drop dwave-drivers (as they're already deprecated).~
• [x] ~Rebase after https://github.com/dwavesystems/dwave-system/pull/543 is merged.~

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.65%. Comparing base (de436db) to head (2c12f63). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #541 +/- ## ========================================== - Coverage 88.95% 85.65% -3.31% ========================================== Files 20 20 Lines 1585 1582 -3 ========================================== - Hits 1410 1355 -55 - Misses 175 227 +52 ```

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[randomir]

Ready for review. Trivial PR after we dropped dwave-drivers. Tagging @JoelPasvolsky for his input on doctests updated for numpy 2.