packaging: don't crash building wix with python3.6 and earlier

`capture_output` was added in 3.7.  I was tempted to just check and abort in
build.py, since Windows doesn't have the Linux problem where some distros only
ship an older python.  But this is in a library that could be used elsewhere in
the future.

contrib/packaging/hgpackaging/util.py

@@ -142,11 +142,9 b' import platform; print("%s:%s" % (platfo'
 def python_exe_info(python_exe: pathlib.Path):
     """Obtain information about a Python executable."""
 
-    res = subprocess.run(
-        [str(python_exe), '-c', PRINT_PYTHON_INFO],
-        capture_output=True, check=True)
+    res = subprocess.check_output([str(python_exe), '-c', PRINT_PYTHON_INFO])
 
-    arch, version = res.stdout.decode('utf-8').split(':')
+    arch, version = res.decode('utf-8').split(':')
 
     version = distutils.version.LooseVersion(version)