run-tests: stuff a `python3.exe` into the test bin directory on Windows Windows doesn't have `python3.exe` as part of the python.org distribution, and that broke every script with a shebang after c102b704edb5. Windows itself provides a `python3.exe` app execution alias[1], but it is some sort of reparse point that MSYS is incapable of handling[2]. When run by MSYS, it simply prints $ python3 -V - Cannot open That in turn caused every `hghave` check, and test that invokes shebang scripts directly, to fail. Rather than try to patch up every script call to be invoked with `$PYTHON` (and regress when non Windows developers forget), copying the executable into the test binary directory with the new name just works. Since this directory is prepended to the system PATH value, it also overrides the broken execution alias. (The `_tmpbindir` is used instead of `_bindir` because the latter causes python3.exe to be copied into the repo next to hg.exe when `test-run-tests.t` runs. Something runs with this version of the executable and subsequent runs of `run-tests.py` inside `test-run-tests.t` try to copy over it while it is in use, and fail. This avoids the failures and the clutter.) I didn't conditionalize this on py3 because `python3.exe` needs to be present (for the shebangs) even when running py2 tests. It shouldn't matter to these simple scripts, and I think the intention is to make the test runner use py3 always, even if testing a py2 build. For now, still supporting py2 is helping to clean up the mess that is py3 tests. [1] https://stackoverflow.com/a/57168165 [2] https://stackoverflow.com/questions/59148628/solved-unable-to-run-python-3-7-on-windows-10-permission-denied#comment104524397_59148666 Differential Revision: https://phab.mercurial-scm.org/D9543