Update __init__.py of SASL to catch ImportErrors in case botocore is not installed (dpkp#175)

Closes kafka-python-ng#174.

Author: wbarnha

kafka/sasl/__init__.py

@@ -1,18 +1,23 @@
 import logging
 
-from kafka.sasl import gssapi, oauthbearer, plain, scram, msk
-
-log = logging.getLogger(__name__)
+from kafka.sasl import gssapi, oauthbearer, plain, scram
 
 MECHANISMS = {
     'GSSAPI': gssapi,
     'OAUTHBEARER': oauthbearer,
     'PLAIN': plain,
     'SCRAM-SHA-256': scram,
     'SCRAM-SHA-512': scram,
-    'AWS_MSK_IAM': msk,
 }
 
+try:
+    from kafka.sasl import msk
+    MECHANISMS['AWS_MSK_IAM'] = msk
+except ImportError:
+    pass
+
+log = logging.getLogger(__name__)
+
 
 def register_mechanism(key, module):
     """