py: migrate to base64.b64encode for Python 3.3 support

Also decodes string as UTF-8 afterwards.

Fixes #8539

Signed-off-by: Andreas Tolfsen <ato@mozilla.com>

Author: jxiaodev

py/selenium/webdriver/firefox/firefox_profile.py

@@ -173,7 +173,7 @@ def encoded(self):
                 filename = os.path.join(base, fyle)
                 zipped.write(filename, filename[path_root:])
         zipped.close()
-        return base64.encodestring(fp.getvalue())
+        return base64.b64encode(fp.getvalue()).decode('UTF-8')
 
     def set_proxy(self, proxy):
         import warnings