deps: pin black==26.5.1 and mypy==2.3.0 to match TSC

[jacalata]

Summary

The two Tableau-owned Python packages (tabcmd and tableauserverclient) were on divergent dev-tool versions:

	tabcmd (before)	tabcmd (after)	TSC
black	`>=22,<23`	`==26.5.1`	`==26.5.1`
mypy	unpinned	`==2.3.0`	`==2.3.0`

Anyone working on both repos (porting fixes back and forth) has to keep swapping black versions in their local env. Match TSC's pins on the tabcmd side; the older repo of the two is the outlier.

Consequential changes from the version bump

• black 26 reformats 9 files (whitespace around decorators, blank-line policy after imports). No behaviour change; the diffs are all whitespace / trailing-comma noise.
• black 26 also required bumping the `[tool.black]` `target-version` — switched to py310-py314 to match tabcmd's `requires-python = ">=3.10"` and `classifiers` list (was py310-py311 only).
• Dropped the `required-version = 22` line from `[tool.black]`; the versioned pin in `[test]` is enough, and TSC doesn't use one either.
• mypy 2.3 caught one pre-existing issue: `tabcmd/version.py` re-imported `version` under the same name inside a fallback branch (`from tabcmd._version import version`), which shadowed the `version: str = "unknown"` annotation. Renamed the fallback import to `_pyinstaller_version` and reassigned. No behavioural change (same value ends up in `version`).

Test plan

• `black . --check` passes under black 26.5.1
• `mypy tabcmd tests` passes with 0 errors under mypy 2.3.0 (was 1 error)
• Full unit-test suite: 337 passed, 2 skipped

🤖 Generated with Claude Code

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tabcmd
__main__.py	12	12	12	0%
_version.py	11	11	11	0%
tabcmd.py	15	15	15	0%
version.py	10	6	6	40%
tabcmd/commands
commands.py	10	10	10	0%
constants.py	77	18	18	77%
server.py	135	18	18	87%
tabcmd/commands/auth
session.py	394	50	50	87%
tabcmd/commands/datasources_and_workbooks
datasources_and_workbooks_command.py	157	18	18	89%
datasources_workbooks_views_url_parser.py	142	5	5	96%
delete_command.py	60	16	16	73%
export_command.py	120	25	25	79%
get_url_command.py	127	47	47	63%
publish_command.py	123	28	28	77%
runschedule_command.py	21	7	7	67%
tabcmd/commands/extracts
create_extracts_command.py	42	8	8	81%
decrypt_extracts_command.py	27	2	2	93%
delete_extracts_command.py	37	6	6	84%
encrypt_extracts_command.py	27	2	2	93%
extracts.py	20	2	2	90%
reencrypt_extracts_command.py	27	2	2	93%
refresh_extracts_command.py	48	10	10	79%
tabcmd/commands/group
create_group_command.py	29	5	5	83%
delete_group_command.py	27	2	2	93%
tabcmd/commands/project
create_project_command.py	46	8	8	83%
delete_project_command.py	35	4	4	89%
publish_samples_command.py	30	4	4	87%
tabcmd/commands/site
create_site_command.py	34	5	5	85%
delete_site_command.py	27	2	2	93%
edit_site_command.py	38	2	2	95%
list_command.py	77	12	12	84%
list_sites_command.py	29	2	2	93%
tabcmd/commands/user
add_users_command.py	29	5	5	83%
create_site_users.py	58	11	11	81%
create_users_command.py	59	9	9	85%
delete_site_users_command.py	43	5	5	88%
user_data.py	222	31	31	86%
tabcmd/execution
_version.py	2	2	2	0%
global_options.py	125	8	8	94%
localize.py	66	11	11	83%
logger_config.py	60	6	6	90%
tabcmd_controller.py	42	7	7	83%
TOTAL	2874	459	84%