warn users of crab-dev. Fix #5134 (#5136)

* warn users of crab-dev. Fix #5134

* better message

bin/crab.py

@@ -9,6 +9,7 @@
 from __future__ import print_function
 from __future__ import division
 import sys
+import os
 if sys.version_info < (2, 6):
     print('\nError: using a version of python < 2.6. Exiting...\n')
     sys.exit()
@@ -19,6 +20,12 @@
     print('pycurl version is: %s\n' % pycurl.version)
     sys.exit()
 
+if 'crab-dev' in __file__:
+    print('BEWARE: this is the development version of CRAB Client.\nBe sure to have a good reason for using it\n')
+    # following is temporary, to be removed in January/February 2022
+    if os.getenv('CMSSW_VERSION').split('_')[1] == '12':
+        print('NOTE: THERE IS NO NEED ANYMORE TO USE crab-dev FOR CMSSW_12. GO BACK TO PLAIN crab\n')
+
 import logging
 import logging.handlers
 import re