Fix "memory leak" in python tests (#652)

OOMs on RPi (1GB)
diff --git a/python/tests/compressor_test.py b/python/tests/compressor_test.py
index 22e3bc5..2d47919 100644
--- a/python/tests/compressor_test.py
+++ b/python/tests/compressor_test.py
@@ -17,6 +17,9 @@
 
     CHUNK_SIZE = 2048
 
+    def tearDown(self):
+        self.compressor = None
+
     def _check_decompression(self, test_data):
         # Write decompression to temp file and verify it matches the original.
         temp_uncompressed = _test_utils.get_temp_uncompressed_name(test_data)
diff --git a/python/tests/decompressor_test.py b/python/tests/decompressor_test.py
index afa127f..99667bc 100644
--- a/python/tests/decompressor_test.py
+++ b/python/tests/decompressor_test.py
@@ -21,6 +21,9 @@
     def setUp(self):
         self.decompressor = brotli.Decompressor()
 
+    def tearDown(self):
+        self.decompressor = None
+
     def _check_decompression(self, test_data):
         # Verify decompression matches the original.
         temp_uncompressed = _test_utils.get_temp_uncompressed_name(test_data)