cli: add zstd import fallback via zstandard for Python < 3.14

CHANGELOG.md

@@ -8,6 +8,9 @@ All notable changes to this project will be documented in this file.
 
 ### Changed
 
+- zstandard fallback import added for python <3.14
+- Zstd libraries are optional dependencies
+
 ### Fixed
 
 ### Removed
@@ -40,7 +43,7 @@ boolean options, all defaulting to `True`:
   - The JSON output file is no longer generated by default.
   - The `--output <PATH>` option have been renamed to `--json-output <PATH>`
   - The `--json-output <PATH>` option must now be explicitly specified to enable JSON file generation.
-  
+
 
 ### Fixed
 

pyproject.toml

@@ -20,8 +20,12 @@ classifiers = [
     "Topic :: Software Development :: Build Tools",
 ]
 
-dependencies = [
-    "backports.zstd; python_version < '3.14'"
+[project.optional-dependencies]
+zstd = [
+    "backports.zstd;python_version<'3.14'",
+]
+zstandard = [
+    "zstandard;python_version<'3.14'",
 ]
 
 [project.scripts]

src/spdx_diff/cli.py

@@ -17,9 +17,15 @@
 from typing import Any
 
 if sys.version_info >= (3, 14):
-    from compression import zstd
+    from compression.zstd import open as open_zstd
 else:
-    from backports import zstd
+    try:
+        from backports.zstd import open as open_zstd  # type: ignore[import-not-found]
+    except ImportError:
+        try:
+            from zstandard import open as open_zstd  # type: ignore[import-not-found]
+        except ImportError:
+            open_zstd = None
 
 from . import __version__
 
@@ -61,7 +67,12 @@ def _parse(self, json_path: pathlib.Path) -> None:
 
         try:
             if json_path.suffix == ".zst":
-                with zstd.open(json_path, "rt") as f:
+                if open_zstd is None:
+                    raise RuntimeError(
+                        "Zstd support is not available."
+                        "Please install the 'zstandard' package."
+                    )
+                with open_zstd(json_path, "rb") as f:
                     data = json.load(f)
             else:
                 with json_path.open(encoding="utf-8") as f: