bpo-34901: add isolated (-I) flag to sys.flags (GH-9708)

https://bugs.python.org/issue34901

Doc/library/sys.rst

@@ -345,6 +345,7 @@ always available.
    :const:`debug`                :option:`-d`
    :const:`inspect`              :option:`-i`
    :const:`interactive`          :option:`-i`
+   :const:`isolated`             :option:`-I`
    :const:`optimize`             :option:`-O` or :option:`-OO`
    :const:`dont_write_bytecode`  :option:`-B`
    :const:`no_user_site`         :option:`-s`
@@ -367,6 +368,9 @@ always available.
    .. versionchanged:: 3.3
       Removed obsolete ``division_warning`` attribute.
 
+   .. versionchanged:: 3.4
+      Added ``isolated`` attribute for :option:`-I` ``isolated`` flag.
+
    .. versionchanged:: 3.7
       Added ``dev_mode`` attribute for the new :option:`-X` ``dev`` flag
       and ``utf8_mode`` attribute for the new  :option:`-X` ``utf8`` flag.