doc build is broken with pillow>=10.0.0

[Takashi Kajinami]

Description
===========
Since pillow in upper-constraints was bumped to >=10.0.0, doc build (tox -e docs) consistently fails with the following error.

```
$ tox -e docs
...
done
WARNING: dot code 'seqdiag {\n API; Conductor; Scheduler; Source; Destination;\n edge_length = 300;\n span_height = 15;\n activation = none;\n default_note_color = white;\n\n API ->> Conductor [label = "cast", note = "resize_instance/migrate_server"];\n Conductor => Scheduler [label = "MigrationTask", note = "select_destinations"];\n Conductor -> Conductor [label = "TargetDBSetupTask"];\n Conductor => Destination [label = "PrepResizeAtDestTask", note = "prep_snapshot_based_resize_at_dest"];\n Conductor => Source [label = "PrepResizeAtSourceTask", note = "prep_snapshot_based_resize_at_source"];\n Conductor => Destination [label = "FinishResizeAtDestTask", note = "finish_snapshot_based_resize_at_dest"];\n Conductor -> Conductor [label = "FinishResizeAtDestTask", note = "update instance mapping"];\n}': 'ImageDraw' object has no attribute 'textsize'
WARNING: dot code 'seqdiag {\n API; Conductor; Source;\n edge_length = 300;\n span_height = 15;\n activation = none;\n default_note_color = white;\n\n API ->> Conductor [label = "cast (or call if deleting)", note = "confirm_snapshot_based_resize"];\n\n // separator to indicate everything after this is driven by ConfirmResizeTask\n === ConfirmResizeTask ===\n\n Conductor => Source [label = "call", note = "confirm_snapshot_based_resize_at_source"];\n Conductor -> Conductor [note = "hard delete source cell instance"];\n Conductor -> Conductor [note = "update target cell instance status"];\n\n}': 'ImageDraw' object has no attribute 'textsize'
WARNING: dot code 'seqdiag {\n API; Conductor; Source; Destination;\n edge_length = 300;\n span_height = 15;\n activation = none;\n default_note_color = white;\n\n API ->> Conductor [label = "cast", note = "revert_snapshot_based_resize"];\n\n // separator to indicate everything after this is driven by RevertResizeTask\n === RevertResizeTask ===\n\n Conductor -> Conductor [note = "update records from target to source cell"];\n Conductor -> Conductor [note = "update instance mapping"];\n Conductor => Destination [label = "call", note = "revert_snapshot_based_resize_at_dest"];\n Conductor -> Conductor [note = "hard delete target cell instance"];\n Conductor => Source [label = "call", note = "finish_revert_snapshot_based_resize_at_source"];\n\n}': 'ImageDraw' object has no attribute 'textsize'
WARNING: dot code 'seqdiag {\n API; Conductor; Scheduler; Source; Destination;\n edge_length = 300;\n span_height = 15;\n activation = none;\n default_note_color = white;\n\n API -> Conductor [label = "cast", note = "resize_instance/migrate_server"];\n Conductor => Scheduler [label = "call", note = "select_destinations"];\n Conductor -> Destination [label = "cast", note = "prep_resize"];\n Source <- Destination [label = "cast", leftnote = "resize_instance"];\n Source -> Destination [label = "cast", note = "finish_resize"];\n}': 'ImageDraw' object has no attribute 'textsize'
WARNING: dot code 'seqdiag {\n API; Source;\n edge_length = 300;\n span_height = 15;\n activation = none;\n default_note_color = white;\n\n API -> Source [label = "cast (or call if deleting)", note = "confirm_resize"];\n}': 'ImageDraw' object has no attribute 'textsize'
WARNING: dot code 'seqdiag {\n API; Source; Destination;\n edge_length = 300;\n span_height = 15;\n activation = none;\n default_note_color = white;\n\n API -> Destination [label = "cast", note = "revert_resize"];\n Source <- Destination [label = "cast", leftnote = "finish_revert_resize"];\n}': 'ImageDraw' object has no attribute 'textsize'
WARNING: dot code 'actdiag {\n build-spec -> send-spec -> send-reqs -> query -> return-rps ->\n create -> filter -> claim -> return-hosts -> send-hosts;\n\n lane conductor {\n label = "Conductor";\n build-spec [label = "Build request spec object", height = 38];\n send-spec [label = "Submit request spec to scheduler", height = 38];\n send-hosts [label = "Submit list of suitable hosts to target cell", height = 51];\n }\n\n lane scheduler {\n label = "Scheduler";\n send-reqs [label = "Submit resource requirements to placement", height = 64];\n create [label = "Create a HostState object for each RP returned from Placement", height = 64];\n filter [label = "Filter and weigh results", height = 38];\n return-hosts [label = "Return a list of selected host & alternates, along with their allocations, to the conductor", height = 89];\n }\n\n lane placement {\n label = "Placement";\n query [label = "Query to determine the RPs representing compute nodes to satisfy requirements", height = 64];\n return-rps [label = "Return list of resource providers and their corresponding allocations to scheduler", height = 89];\n claim [label = "Create allocations against selected compute node", height = 64];\n }\n}': 'ImageDraw' object has no attribute 'textsize'

Sphinx parallel build error:
AssertionError: len(context) = 1
```

pillow 10.0.0 removed the deprecated interfaces including textsize which is seen in the error.

https://pillow.readthedocs.io/en/stable/releasenotes/10.0.0.html#font-size-and-offset-methods

Steps to reproduce
==================
- Build documentation using the current master
 $ tox -e docs

Expected result
===============
Build completes without error

Actual result
=============
Build fails with the error described above

Environment
===========
N/A

Logs & Configs
==============
N/A

[Takashi Kajinami (kajinamit)]

We can probably attempt to fix https://opendev.org/openstack/nova/src/branch/master/doc/source/conf.py#L260 . Note that the same problem was reported in upstream blockdiag[1].

[1] https://github.com/blockdiag/blockdiag/issues/178

One important note is that blockdiag looks unmaintained[2]. NO PR has been merged since 2021. We probably have to even replace it by something else.

[2] https://github.com/blockdiag/blockdiag/issues/177

[Takashi Kajinami (kajinamit)]

In the mean time we downgrade pillow https://review.opendev.org/c/openstack/requirements/+/906262