packaging: switch centos 7 packaging to python 3 Differential Revision: https://phab.mercurial-scm.org/D9293

packaging: remove centos5 and centos6 support Differential Revision: https://phab.mercurial-scm.org/D9292

test-filecache: use sys.executable to call python As was mentioned in c102b704edb5, test scripts calling 'python' or 'python3' might use the wrong python. For test-filecache.py, this causes a failed test on CentOS 7. Differential Revision: https://phab.mercurial-scm.org/D9295

make: add a pyoxidizer target Differential Revision: https://phab.mercurial-scm.org/D9291

pyoxidizer: switch to modern config using run_command instead of run_mode Differential Revision: https://phab.mercurial-scm.org/D9290

pyoxidizer: default to one-file binary on non-Windows platforms Windows has some extra constraints that require a multi-file install, but we expect folks to use an MSI or similar installer there so it's less of a big deal. Differential Revision: https://phab.mercurial-scm.org/D9289

global: use python3 in shebangs Python 3 is the future. We want Python scripts to be using Python 3 by default. This change updates all `#!/usr/bin/env python` shebangs to use `python3`. Does this mean all scripts use or require Python 3: no. In the test environment, the `PATH` environment variable in tests is updated to guarantee that the Python executable used to run run-tests.py is used. Since test scripts all now use `#!/usr/bin/env python3`, we had to update this code to install a `python3` symlink instead of `python`. It is possible there are some random scripts now executed with the incorrect Python interpreter in some contexts. However, I would argue that this was a pre-existing bug: we should almost always be executing new Python processes using the `sys.executable` from the originating Python script, as `python` or `python3` won't guarantee we'll use the same interpreter. Differential Revision: https://phab.mercurial-scm.org/D9273

tests: use python from environment in test-parseindex2.py Without this, the test starts failing with D9273 (the change to `pyexename` to be specific). Differential Revision: https://phab.mercurial-scm.org/D9286