Use pytest.importorskip for C-extension tests

As these bits may or may not be built depending on whether C-extensions
are supported, have pytest skip them if they are not `import`able.

Author: jakirkham

numcodecs/tests/test_blosc.py

@@ -6,6 +6,9 @@
 import pytest
 
 
+pytest.importorskip("numcodecs.blosc")
+
+
 from numcodecs import blosc
 from numcodecs.blosc import Blosc
 from numcodecs.tests.common import (check_encode_decode,

numcodecs/tests/test_lz4.py

@@ -2,6 +2,10 @@
 
 
 import numpy as np
+import pytest
+
+
+pytest.importorskip("numcodecs.lz4")
 
 
 from numcodecs.lz4 import LZ4

numcodecs/tests/test_zstd.py

@@ -2,6 +2,10 @@
 
 
 import numpy as np
+import pytest
+
+
+pytest.importorskip("numcodecs.zstd")
 
 
 from numcodecs.zstd import Zstd