Fix an ImportError w/Blosc on Android (#284)

Android does not have a functioning semaphore system (actually it's
    bionic-C that doesn't...). When trying to import numcodecs.blosc on
Android, an ImportError exception was thrown - which was unhandled.

Author: danieldjewell

numcodecs/blosc.pyx

@@ -79,6 +79,8 @@ try:
     mutex = multiprocessing.Lock()
 except OSError:
     mutex = None
+except ImportError:
+    mutex = None
 
 # store ID of process that first loads the module, so we can detect a fork later
 _importer_pid = os.getpid()