packaging: don't use plain 'python' if another python has been specified
Before this change, packaging on CentOS 8 failed because 'python'
is used instead of 'python3'.
Change was tested with:
- make docker-centos7
- make docker-centos8
- make docker-ubuntu-bionic
Differential Revision: https://phab.mercurial-scm.org/D9464
#require black test-repo
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`