misc/py-litellm: update to 1.28.7

Changelog:

• https://github.com/BerriAI/litellm/releases/tag/v1.27.15
• https://github.com/BerriAI/litellm/releases/tag/v1.28.0
• https://github.com/BerriAI/litellm/releases/tag/v1.28.1
• https://github.com/BerriAI/litellm/releases/tag/v1.28.2
• https://github.com/BerriAI/litellm/releases/tag/v1.28.3
• https://github.com/BerriAI/litellm/releases/tag/v1.28.4
• https://github.com/BerriAI/litellm/releases/tag/v1.28.6
• https://github.com/BerriAI/litellm/releases/tag/v1.28.7

Reported by: Repology