Fix Windows path separator issue in constraints reference

On Windows, os.path.relpath() returns paths with backslashes,
but pip expects forward slashes in constraint file references.

This commit normalizes all constraint paths to use forward slashes
by adding .replace("\\", "/") after path calculation.

This fixes CI test failures on Windows and macOS platforms.

Related to #22 and #25

Author: jensens

CHANGES.md

@@ -2,7 +2,7 @@
 
 ## 4.1.2 (unreleased)
 
-- Fix #22 and #25: Constraints file path in requirements-out is now correctly calculated as a relative path from the requirements file's directory. This allows requirements and constraints files to be in different directories. Previously, the path was written from the config file's perspective, causing pip to fail when looking for the constraints file.
+- Fix #22 and #25: Constraints file path in requirements-out is now correctly calculated as a relative path from the requirements file's directory. This allows requirements and constraints files to be in different directories. Previously, the path was written from the config file's perspective, causing pip to fail when looking for the constraints file. On Windows, paths are now normalized to use forward slashes for pip compatibility.
   [jensens]
 
 - Fix #53: Per-package target setting now correctly overrides default-target when constructing checkout paths.

src/mxdev/processing.py

@@ -276,10 +276,13 @@ def write(state: State) -> None:
             # Calculate relative path from requirements directory to constraints file
             try:
                 constraints_ref = os.path.relpath(const_path, req_path.parent)
+                # Convert backslashes to forward slashes for pip compatibility
+                # pip expects forward slashes even on Windows
+                constraints_ref = constraints_ref.replace("\\", "/")
             except ValueError:
                 # On Windows, relpath can fail if paths are on different drives
-                # In that case, use absolute path
-                constraints_ref = str(const_path.absolute())
+                # In that case, use absolute path with forward slashes
+                constraints_ref = str(const_path.absolute()).replace("\\", "/")
 
             fio.write("#" * 79 + "\n")
             fio.write("# mxdev combined constraints\n")