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

[jonatack]

by adding to each a python shebang and changing their file permissions from 644 to 755, as mentioned in #1734 (comment).

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:

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

After:

$ ./reference.py 

[andrewtoth]

ACK cce668d