boards.txt.py - use keywords for re.sub count=... and flags=...

https://docs.python.org/3/library/re.html#re.sub
> Deprecated since version 3.13:
> Passing count and flags as positional arguments is deprecated.
> In future Python versions they will be keyword-only parameters.

Author: mcspr

tools/boards.txt.py

@@ -1877,7 +1877,7 @@ def package ():
     substitution += ',\n'.join(board_items)
     substitution += '\n          ],'
 
-    newfilestr = re.sub(r'"boards":[^\]]*\],', substitution, filestr, re.MULTILINE)
+    newfilestr = re.sub(r'"boards":[^\]]*\],', substitution, count=filestr, flags=re.MULTILINE)
 
     # To get consistent indent/formatting read the JSON and write it out programmatically
     if packagegen: