fix: convert unicode to str for py2

Author: satra

nipype/interfaces/base.py

@@ -1286,10 +1286,10 @@ def _process(drain=0):
         result['stderr'] = []
         result['merged'] = ''
     if sys.version_info < (3,):
-        runtime.stderr = '\n'.join(result['stderr']).decode('utf-8')
-        runtime.stdout = '\n'.join(result['stdout']).decode('utf-8')
+        runtime.stderr = str('\n'.join(result['stderr']).decode('utf-8'))
+        runtime.stdout = str('\n'.join(result['stdout']).decode('utf-8'))
         if isinstance(result['merged'], list):
-            runtime.merged = [val.decode('utf-8') for val in result['merged']]
+            runtime.merged = [str(val.decode('utf-8')) for val in result['merged']]
         else:
             runtime.merged = result['merged']
     else: