[CMAKE] Migrate the python header generation scripts to python 3

Summary:
The shebang has already been updated, but the scripts would fail with
python3. The read() function now returns a byte array when the file is
opened in binary mode, so the ord() is no longer needed.

The CMakeLists.txt files are updated to use the correct interpreter.

Test Plan:

ninja check
ninja bitcoin-bench

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4143