Close python#19284: Handle -R properly in flag helper

Previously, the -R option would be specified multiple times
if PYTHONHASHSEED was set.

Author: ncoghlan

Lib/subprocess.py

@@ -506,6 +506,8 @@ def _args_from_interpreter_flags():
     for flag, opt in flag_opt_map.items():
         v = getattr(sys.flags, flag)
         if v > 0:
+            if flag == 'hash_randomization':
+                v = 1 # Handle specification of an exact seed
             args.append('-' + opt * v)
     for opt in sys.warnoptions:
         args.append('-W' + opt)