tests/extmod/re_sub.py: Fix test execution on Python 3.13.

This commit fixes a test failure for `extmod/re_sub.py` where the code,
whilst being correct, would not make the test pass due to a newer
Python version than expected.

On Python 3.13, running `tests/extmod/re_sub.py` would yield a
deprecation warning about `re.sub` not providing the match count as a
keyword parameter.  This warning would be embedded in the expected test
result and thus the test would always fail.

Co-authored-by: stijn <stijn@ignitron.net>
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

tests/extmod/re_sub.py

@@ -10,6 +10,8 @@ except AttributeError:
     print("SKIP")
     raise SystemExit
 
+import sys
+
 
 def multiply(m):
     return str(int(m.group(0)) * 2)
@@ -47,6 +49,10 @@ print(re.sub("(abc)", r"\g<1>\g<1>", "abc"))
 print(re.sub("a", "b", "c"))
 
 # with maximum substitution count specified
+if sys.implementation.name != "micropython":
+    # On CPython 3.13 and later the substitution count must be a keyword argument.
+    print(re.sub("a", "b", "1a2a3a", count=2))
+else:
     print(re.sub("a", "b", "1a2a3a", 2))
 
 # invalid group