Fix 2 places where encoding is not specified

- In `pip/_internal/configuration.py`, explicitly use locale encoding to parse configuration file
- In `pip/_internal/build_env.py`, use UTF-8 when generating `sitecustomize.py`

Fixes #10590.

src/pip/_internal/build_env.py

@@ -93,7 +93,9 @@ class BuildEnvironment:
         self._site_dir = os.path.join(temp_dir.path, "site")
         if not os.path.exists(self._site_dir):
             os.mkdir(self._site_dir)
-        with open(os.path.join(self._site_dir, "sitecustomize.py"), "w") as fp:
+        with open(
+            os.path.join(self._site_dir, "sitecustomize.py"), "w", encoding="utf-8"
+        ) as fp:
             fp.write(
                 textwrap.dedent(
                     """

src/pip/_internal/configuration.py

@@ -266,14 +266,13 @@ class Configuration:
         # Doing this is useful when modifying and saving files, where we don't
         # need to construct a parser.
         if os.path.exists(fname):
+            locale_encoding = locale.getpreferredencoding(False)
             try:
-                parser.read(fname)
+                parser.read(fname, encoding=locale_encoding)
             except UnicodeDecodeError:
                 # See https://github.com/pypa/pip/issues/4963
                 raise ConfigurationFileCouldNotBeLoaded(
-                    reason="contains invalid {} characters".format(
-                        locale.getpreferredencoding(False)
-                    ),
+                    reason="contains invalid {} characters".format(locale_encoding),
                     fname=fname,
                 )
             except configparser.Error as error: