feat(python): add license support for `requirement.txt` files

[DmitriyLewen]

Description

Add license support for requirement.txt files.

➜  trivy -q fs --scanners license ./pkg/fanal/analyzer/language/python/pip/testdata/happy 

requirements.txt (license)

Total: 1 (UNKNOWN: 1, LOW: 0, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

┌─────────┬─────────────┬────────────────┬──────────┐
│ Package │   License   │ Classification │ Severity │
├─────────┼─────────────┼────────────────┼──────────┤
│ Flask   │ BSD License │ Non Standard   │ UNKNOWN  │
└─────────┴─────────────┴────────────────┴──────────┘

Related issues

• Close #6703

Checklist

• [x] I've read the guidelines for contributing to this repository.
• [x] I've followed the conventions in the PR title.
• [x] I've added tests that prove my fix is effective or that my feature works.
• [x] I've updated the documentation with the relevant information (if needed).
• [ ] I've added usage information (if the PR introduces new options)
• [x] I've included a "before" and "after" example to the description (if the PR is a user interface change).