rpcuser.py: Use 'python' not 'python2'
Summary:
Note that rpcuser.py seems to handle either version when called explicitly like python2 rpcuser.py / python3 rpcuser.py. This change allows the user's python to be used to interpret the script.
Backport of core PR11830
https://github.com/bitcoin/bitcoin/pull/11830/files
Test Plan:
python2 share/rpcuser/rpcuser.py foo python3 share/rpcuser/rpcuser.py foo
Should return the rpcauth string without error
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3553