tests/testlib/common.sh

automation: delete code related to Python 2.7 support

The building of Inno and WiX installers took a python_version argument
that allowed us to specify "2" or "3" for the major Python version. Since
we no longer support Python 2, we can delete this argument and everything
feeding into it.

Differential Revision: https://phab.mercurial-scm.org/D12264

mkcommit() {
   name="$1"
   shift
   echo "$name" > "$name"
   hg add "$name"
   hg ci -m "$name" "$@"
}