fix: handle UTF-16 and BOM encoded requirements.txt files

[sliortega295-ops]

Summary

Fixes #1393.

When a requirements.txt file is saved with UTF-16 encoding (e.g., by VS Code), deptry fails to parse it because the BOM (Byte Order Mark) characters are misinterpreted as part of the package name.

Changes

• Modified get_dependencies_from_requirements_file() in requirements_files.py to read file bytes first and detect encoding via BOM before decoding
• Added _read_requirements_file() helper that handles:
  • UTF-16 LE (BOM: \xff\xfe)
  • UTF-16 BE (BOM: \xfe\xff)
  • UTF-8 with BOM (BOM: \xef\xbb\xbf)
  • Plain UTF-8 (no BOM, default)

Testing

• Added 3 new unit tests for UTF-16 LE, UTF-16 BE, and UTF-8 with BOM encoded requirements.txt files
• All 10 unit tests pass (7 existing + 3 new)
• No regressions in existing tests