Deprecate sphinx.util.osutil.movefile() in favor of os.replace()

The utility function movefile() was added in
677d096393 to handle existing files on
Windows. Since Python 3.3, the stdlib function os.replace() fills this
role.

doc/extdev/deprecated.rst

@@ -77,6 +77,11 @@ The following is a list of deprecated interfaces.
      - 5.0
      - ``sphinx.pycode.ModuleAnalyzer.analyze()``
 
+   * - ``sphinx.util.osutil.movefile()``
+     - 3.4
+     - 5.0
+     - ``os.replace()``
+
    * - ``sphinx.util.requests.is_ssl_error()``
      - 3.4
      - 5.0