work around Python 3.11 IntEnum issue; update FastIntFlag

work around Python 3.11 IntEnum issue; update FastIntFlag

in [1], Python 3.11 seems to have changed the behavior of
IntEnum.  We didn't notice this because we have our own
workaround class already, but typing did.   Ensure we remain
compatible with IntFlag.

This change also modifies FastIntFlag to no longer use
global symbols; this is unnecessary as we assign FastIntFlag
members explicitly.  Use of ``symbol()`` should probably
be phased out.

[1] https://github.com/python/cpython/issues/99304
Fixes: #8783
Change-Id: I8ae2e871ff1467ae5ca1f63e66b5dae45d4a6c93