packaging: use /usr/bin/python3 shebang for scripts in Debian
"hg" script is already correct because it is handled by setup.py but
"hg-ssh" will be rewritten by dh_python into "/usr/bin/python" which is
not wanted as we target Python 3. By passing --shebang=/usr/bin/python3
to dh_python3, we force shebangs to be set with this value.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).