Make TaskVineExecutor importable when ndcctools is not available (#2909)

Fixes issue #2903

parsl/executors/taskvine/factory.py

@@ -3,13 +3,22 @@
 from parsl.process_loggers import wrap_with_logs
 from parsl.executors.taskvine.errors import TaskVineFactoryFailure
 
-from ndcctools.taskvine import Factory
+# This try except clause prevents import errors
+# when TaskVine is not used in Parsl.
+try:
+    from ndcctools.taskvine import Factory
+    taskvine_available = True
+except ImportError:
+    taskvine_available = False
 
 logger = logging.getLogger(__name__)
 
 
 @wrap_with_logs
 def _taskvine_factory(should_stop, factory_config):
+    if not taskvine_available:
+        logger.debug("TaskVine package cannot be found. Please install the ndcctools package.")
+        return
     logger.debug("Starting TaskVine factory process")
 
     try:

parsl/executors/taskvine/manager_config.py

@@ -2,7 +2,12 @@
 from dataclasses import dataclass
 from typing import Optional
 
-from ndcctools.taskvine.cvine import VINE_DEFAULT_PORT
+# This try except clause prevents import errors
+# when TaskVine is not used in Parsl.
+try:
+    from ndcctools.taskvine.cvine import VINE_DEFAULT_PORT
+except ImportError:
+    VINE_DEFAULT_PORT = 0   # use any available port.
 
 
 @dataclass