build: add bg-mpl-stylesheet to have better plot

[stevenhua0320]

@sbillinge ready to review, now with the bg-mpl-stysheet included, we could have better color effect on the plot. But we might need to release a new version of bg-mpl-stylesheet to python 3.14. Closes#146

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.47%. Comparing base (d9e435d) to head (70bbcf4).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #180 +/- ##
=======================================
Coverage 76.47% 76.47% =======================================
Files 3 3 Lines 51 51 =======================================
Hits 39 39 Misses 12 12 

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.