BIP374: update reference.py and secp256k1.py to be executable

by adding to each a python shebang and changing their file permissions from 644 to 755, as mentioned in https://github.com/bitcoin/bips/pull/1734#issuecomment-2564476828. These files don't need to be executable in order to run `gen_test_vectors.py` and `run_test_vectors.py` but it is helpful to allow developers to invoke them directly. Before: ```bash $ ./reference.py zsh: permission denied: ./reference.py ``` After: ```bash $ ./reference.py ```

Specification

$ ./reference.py 
zsh: permission denied: ./reference.py

$ ./reference.py