fix python file encoding error on Windows

Author: siradam7th

libcxx/utils/generate_libcxx_cppm_in.py

@@ -13,7 +13,7 @@
 
 
 def write_file(module):
-    with open(libcxx_root / "modules" / f"{module}.cppm.in", "w") as module_cpp_in:
+    with open(libcxx_root / "modules" / f"{module}.cppm.in", "w", encoding='utf-8') as module_cpp_in:
         module_cpp_in.write(
             """\
 // -*- C++ -*-