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.)

mercurial/exewrapper.c

@@ -7,6 +7,7 b''
  GNU General Public License version 2 or any later version.
 */
 
+#include <Python.h>
 #include <stdio.h>
 #include <tchar.h>
 #include <windows.h>
@@ -46,6 +47,10 b' int _tmain(int argc, TCHAR *argv[])'
 	void(__cdecl * Py_SetPythonHome)(TCHAR * home);
 	int(__cdecl * Py_Main)(int argc, TCHAR *argv[]);
 
+#if PY_MAJOR_VERSION >= 3
+	Py_LegacyWindowsStdioFlag = 1;
+#endif
+
 	if (GetModuleFileName(NULL, pyscript, _countof(pyscript)) == 0) {
 		err = "GetModuleFileName failed";
 		goto bail;

tests/run-tests.py

@@ -2640,6 +2640,10 b' class TestRunner(object):'
             self._tmpbindir = self._bindir
             self._pythondir = os.path.join(self._installdir, b"lib", b"python")
 
+        # Force the use of hg.exe instead of relying on MSYS to recognize hg is
+        # a python script and feed it to python.exe.  Legacy stdio is force
+        # enabled by hg.exe, and this is a more realistic way to launch hg
+        # anyway.
         if os.name == 'nt' and not self._hgcommand.endswith(b'.exe'):
             self._hgcommand += b'.exe'