[MNT] remove pytest dependency

[fkiraly]

This removes skbase's dependency on pytest which should not be required.

pytest is still required in the developer dependency set.
Rationale is minimizing dependencies.

E.g., sktime would otherwise incur a new core dependency, pytest - this violates the original intention of this being a pure refactor.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.82%. Comparing base (b27c0be) to head (e991e82).
⚠️ Report is 289 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
=======================================
  Coverage   82.82%   82.82%           
=======================================
  Files          39       39           
  Lines        2795     2795           
=======================================
  Hits         2315     2315           
  Misses        480      480           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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