tests: fix test_vsenv_option with Python 3.11+ on Windows

meson tests enable PYTHONWARNDEFAULTENCODING by default and
make EncodingWarning fatal too.

Starting with Python 3.11 CPython not only warns if no encoding is passed
to open() but also to things like subprocess.check_output(). This made
the call in vsenv.py fail and in turn made test_vsenv_option fail.

check_output() here calls a .bat file which in turn calls vcvars. I don't
know what the encoding is supposed to be used there, so just be explicit
with the locale encoding to silence the warning.

mesonbuild/utils/vsenv.py

@@ -6,6 +6,7 @@ import json
 import pathlib
 import shutil
 import tempfile
+import locale
 
 from .. import mlog
 from .core import MesonException
@@ -93,7 +94,8 @@ def _setup_vsenv(force: bool) -> bool:
     bat_file.write(bat_contents)
     bat_file.flush()
     bat_file.close()
-    bat_output = subprocess.check_output(bat_file.name, universal_newlines=True)
+    bat_output = subprocess.check_output(bat_file.name, universal_newlines=True,
+                                         encoding=locale.getpreferredencoding(False))
     os.unlink(bat_file.name)
     bat_lines = bat_output.split('\n')
     bat_separator_seen = False