Description of problem:
On LDAP integration in keystone, local_id parameter may have bytes data type and it caused an exception because the data type doesn't have encode type.
In the customer's site, the login through horizon always failed with following error in var/log/containers/httpd/keystone/keystone_wsgi_error.log.

~~~
  File "/usr/lib/python3.6/site-packages/keystone/assignment/core.py", line 851, in _list_effective_role_assignments
    group_ids = self._get_group_ids_for_user_id(user_id)
  File "/usr/lib/python3.6/site-packages/keystone/assignment/core.py", line 88, in _get_group_ids_for_user_id
    x in PROVIDERS.identity_api.list_groups_for_user(user_id)]
  File "/usr/lib/python3.6/site-packages/keystone/common/manager.py", line 116, in wrapped
    __ret_val = __f(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 414, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 424, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 1312, in list_groups_for_user
    ref_list, domain_id, driver, mapping.EntityType.GROUP)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 603, in _set_domain_id_and_mapping
    ref, domain_id, driver, entity_type, conf)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 693, in _set_domain_id_and_mapping_for_list
    self._insert_new_public_id(local_entity, ref, driver)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 619, in _insert_new_public_id
    local_entity, public_id)
  File "/usr/lib/python3.6/site-packages/keystone/common/manager.py", line 116, in wrapped
    __ret_val = __f(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/keystone/identity/core.py", line 1465, in create_id_mapping
    public_id = self.driver.create_id_mapping(local_entity, public_id)
  File "/usr/lib/python3.6/site-packages/keystone/identity/mapping_backends/sql.py", line 77, in create_id_mapping
    entity)
  File "/usr/lib/python3.6/site-packages/keystone/identity/id_generators/sha256.py", line 25, in generate_public_ID
    m.update(mapping[key].encode('utf-8'))
AttributeError: 'bytes' object has no attribute 'encode'
~~~

Version-Release number of selected component (if applicable):
RHOSP16.1 keystone

How reproducible:
Always

Steps to Reproduce:
1. Setup RHOSP16.1 with LDAP integration in keystone
2.
3.

Actual results:
The login through horizon always failed.

Expected results:
The login succeeds.

Additional info:

In RHOSP16, python-ldap is updated to python3-ldap-3.1.0-5.el8.x86_64.
From Python3, python distinguish str(sequence of characters) and bytes(sequence of bytes).
For the attribute values, python-ldap uses bytes data type to store the data because attributes may contain any type of data.

c.f. https://www.python-ldap.org/en/python-ldap-3.3.0/bytes_mode.html

That means keystone/identity/id_generators/sha256.py should be
~~~
import hashlib
import six

from keystone.identity import generator


class Generator(generator.IDGenerator):

    def generate_public_ID(self, mapping):
        m = hashlib.sha256()
        for key in sorted(mapping.keys()):
            if six.PY3 and isinstance(mapping[key], six.binary_type):
                m.update(mapping[key])
            else:
                m.update(mapping[key].encode('utf-8'))
        return m.hexdigest()
~~~

This modification works fine on the customer's site.