Explicitly use 'locale' encoding for .pth files in setuptools.namespaces

Author: abravalheri

setuptools/namespaces.py

@@ -2,6 +2,8 @@
 from distutils import log
 import itertools
 
+from .compat import py39
+
 
 flatten = itertools.chain.from_iterable
 
@@ -23,7 +25,8 @@ def install_namespaces(self):
             list(lines)
             return
 
-        with open(filename, 'wt') as f:
+        with open(filename, 'wt', encoding=py39.LOCALE_ENCODING) as f:
+            # Requires encoding="locale" instead of "utf-8" (python/cpython#77102).
             f.writelines(lines)
 
     def uninstall_namespaces(self):