Fix `get_opening_explorer_move` when having the black pieces

[AttackingOrDefending]

Type of pull request:

• [x] Bug fix
• [ ] Feature
• [ ] Other

Description:

The winrate calculation in get_opening_explorer_move was always from whites perspective, so when playing black, it would choose the worst move available.

Related Issues:

fixes #954

Checklist:

• [x] I have read and followed the contribution guidelines.
• [x] I have added necessary documentation (if applicable).
• [x] The changes pass all existing tests.

Screenshots/logs (if applicable):