py3: enable legacy stdio mode in exewrapper This drops the test failure count from 166 to 117. The failures were typically in the form of `hg serve -d` spawning but crashing immediately, leaving clients with "bad http status" lines, connection refusals, and so forth. The underlying message on the server side was either "OSError: [WinError 6] The handle is invalid" or "OSError: [WinError 1] Incorrect function". Additionally, no output was rendered if the pager was activated. Thanks to Yuya for diagnosing the problem. The failure count drops to 107 when PYTHONLEGACYWINDOWSSTDIO=1 is defined in the environment. These failures seem to revolve around the dummyssh server process, and dumbhttp.py. So I'll probably add that to the test runner. One small regression here (only in py3) is that if hg.exe is already built, a messagebox appears when building it again saying that python37.dll can't be loaded. Python3 isn't in PATH by default, and setup.py tries running bare `hg` first. But MSYS prepends '.' to PATH, so it runs the local hg, but can't find the library. It falls back to the python used to invoke setup.py, so ultimately it works. I'm not sure if it's better to strip '.' from PATH or just skip right to `sys.executable hg` on Windows. Also, something seems to be wrong with run-tests._usecorrectpython(). I accidentially left off the 'PYTHON="py -3"' when building (thus making py2 stuff), and yet managed to invoke run-tests.py with "py -3". (And that only had 67 failures.)