Fix constraints path to be relative to requirements file location

[jensens]

Summary

Fixes #22 and #25: Constraints file path in requirements-out is now correctly calculated as a relative path from the requirements file's directory.

Problem

When and are in different directories, the generated requirements file referenced constraints with the wrong path, causing pip to fail.

Example from #22:

[settings]
requirements-out = requirements/plone.txt
constraints-out = requirements/constraints.txt

Before (broken):
requirements/plone.txt contained:

-c requirements/constraints.txt

Pip interprets -c paths relative to the requirements file's directory, so it looked for:

requirements/requirements/constraints.txt  ❌ Wrong!

After (fixed):
requirements/plone.txt now contains:

-c constraints.txt

Because both files are in requirements/, the relative path is just constraints.txt.

Solution

Added path calculation logic in src/mxdev/processing.py:

# Calculate relative path from requirements directory to constraints file
req_path = Path(cfg.out_requirements)
const_path = Path(cfg.out_constraints)
constraints_ref = os.path.relpath(const_path, req_path.parent)

This works for all scenarios:

• Same directory: constraints.txt
• Parent directory: ../constraints.txt
• Subdirectory: subdir/constraints.txt
• Different directories: ../other/constraints.txt
• Different drives (Windows): Falls back to absolute path

Changes

• processing.py: Calculate relative path using os.path.relpath()
• test_processing.py: Added 2 tests for different directory scenarios
• CHANGES.md: Documented the fix

Test Coverage

pytest tests/test_processing.py::test_relative_constraints_path_in_subdirectory -v
pytest tests/test_processing.py::test_relative_constraints_path_different_directories -v

Both tests demonstrate the bug and verify the fix.

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cc @davisagli @gforcada