Make the use of pkg_resources optional

As it's only to enhance the logging, we'll just log the version of
argo_ams_library if the module is available. pkg_resources is
distributed with setuptools, which is commonly installed, but we don't
want to add any dependencies for this "nice to have" functionality.

ssm/ssm2.py

@@ -15,8 +15,6 @@
 """
 from __future__ import print_function
 
-import pkg_resources
-
 from ssm import crypto
 from ssm.message_directory import MessageDirectory
 
@@ -28,6 +26,13 @@
     QueueSimple = None
     Queue = None
 
+try:
+    import pkg_resources
+except ImportError:
+    # pkg_resources is distributed with setuptools, but as it's only used to enhance
+    # the logging, it can be optional.
+    pkg_resources = None
+
 import stomp
 from stomp.exception import ConnectFailedException
 
@@ -551,7 +556,9 @@ def handle_connect(self):
         connect to each in turn until successful.
         """
         if self._protocol == Ssm2.AMS_MESSAGING:
-            log.info("Using AMS version %s", pkg_resources.get_distribution('argo_ams_library').version)
+            if pkg_resources is not None:
+                # We only log the version if pkg_resources is available.
+                log.info("Using AMS version %s", pkg_resources.get_distribution('argo_ams_library').version)
 
             log.info("Will connect to %s", self._brokers[0])