support/testing: sample_python_pybind.py: use python3 shebang

No functional change as we install a python symlink, but use python3 for
consistency with the other scripts.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit ed9288505caa4c3d1c30d5d73ef3189a40ae243a)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/testing/tests/package/sample_python_pybind.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import example
 print(example.add(1, 2))
 print(example.says)