Update boxplot.py
Refactored the code to improve type hinting compatibility  across several functions:
- replaced the | operator with Union for Python versions before 3.10
- added more detailed error messages
- improved logging for edge cases
- redundant logic is simplified, and handling for empty arrays and invalid color arguments
Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.