pybind/rados: define unicode alias

pybind/rados: define unicode alias

for silencing warnings like:

WARNING: Cannot resolve forward reference in type annotations of "rados.Rados.conf_get": name 'unicode' is not defined

because cython < 3.0 with language_level = "3", translates "str" to "unicode"
to be python2 compatible, but we've migrated to python3. and the specified
"language_level" is "3'. see also
https://github.com/cython/cython/issues/1370

Signed-off-by: Kefu Chai <kchai@redhat.com>