rpcgen: use proper operators when comparing types

rpcgen: use proper operators when comparing types

flake8 (run on all python scripts as a part of the syntax checks)
version 6.1.0 (on macOS 14) issued many complaints like this on the
new rpcgen python scripts:

[...]libvirt/scripts/rpcgen/rpcgen/lexer.py:57:17: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`

This patch changes all [type] == [type] to use "is" instead of "==",
and similarly to use "is not" instead of "!=".

(flake8 5.03, e.g. on Fedora 38, is just fine with using "==" and "!=",
but python on both likes "is" and "is not")

Fixes: commit v9.9.0-24-g8ec79e5e14
Fixes: commit v9.9.0-22-gca3f025011
Fixes: commit v9.9.0-21-g031efb691f
Fixes: commit v9.9.0-20-g8c8b97685b
Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>