Support zstd compression via the Python 3.14 compression module

Map --compression values to the compression package submodules so zstd
is available alongside gzip, lzma and bz2. Default remains gzip.

tests/tools_test.py

@@ -1,7 +1,9 @@
 
 import contextlib
 import os
+import pickle
 import shutil
+import tempfile
 import unittest
 import unittest.mock
 
@@ -158,5 +160,25 @@ class LruCacheWithEvictionTestCase(unittest.TestCase):
         self._assert_cache(a, 1, 2, 2)
 
 
+class CompressionTestCase(unittest.TestCase):
+
+    def setUp(self):
+        self.temp_dir = tempfile.mkdtemp()
+
+    def tearDown(self):
+        shutil.rmtree(self.temp_dir)
+
+    def test_open_funcs_round_trip(self):
+        value = {"text": list(range(1000))}
+        for compression in ["gzip", "lzma", "bz2", "zstd", "none"]:
+            with self.subTest(compression=compression):
+                open_func = tools.compression_open_func(compression)
+                path = os.path.join(self.temp_dir, f"cache.{compression}")
+                with open_func(path, "wb") as cache_file:
+                    pickle.dump(value, cache_file)
+                with open_func(path, "rb") as cache_file:
+                    self.assertEqual(pickle.load(cache_file), value)
+
+
 if __name__ == "__main__":
     golden.main()