Fix double use of f.close().

The other one is in a finally block not seen in the diff, which I added
in 3bf86785cd9c (for #10252).

Lib/sysconfig.py

@@ -639,7 +639,6 @@ def get_platform():
                     m = re.search(
                             r'<key>ProductUserVisibleVersion</key>\s*' +
                             r'<string>(.*?)</string>', f.read())
-                    f.close()
                     if m is not None:
                         macrelease = '.'.join(m.group(1).split('.')[:2])
                     # else: fall back to the default behaviour