Ability to define unique columns when creating a table

[sqmyou]

Refs #558.

Summary

Adds a unique= argument for defining UNIQUE constraints when a table is created, mirroring the existing not_null= argument.

db["ExampleTable"].create(
    {"mRID": str, "name": str},
    pk="mRID",
    unique=["mRID"],
    if_not_exists=True,
)

A string names a single column; a tuple names a compound constraint spanning several columns. Both can be combined in one list:

db.table("memberships").create(
    {"account_id": int, "email": str, "role": str},
    unique=["role", ("account_id", "email")],
)

Produces:

CREATE TABLE "memberships" (
   "account_id" INTEGER,
   "email" TEXT,
   "role" TEXT,
   UNIQUE ("role"),
   UNIQUE ("account_id", "email")
)

Where it is accepted

• Database.create_table()
• Table.create()
• Table.transform() - adds the constraint to an existing table; repeating a constraint that is already present is a no-op
• The table-creating methods insert(), insert_all(), upsert() and upsert_all()
• As a table default via db.table(..., unique=[...])

create(..., transform=True) detects a newly requested constraint and applies it.

Also added

• A table.uniques introspection property listing the UNIQUE constraints on a table, mirroring table.checks.
• Documentation for the new argument and property in docs/python-api.rst.

Tests

16 new tests in tests/test_create.py cover single-column, multi-column and compound constraints, constraint enforcement, the bad-column error, persistence via _defaults, transform (including idempotency and renamed columns), transform=True, and the insert/upsert paths.

Full suite: 1513 passed, 16 skipped. black, flake8, mypy, pyright and cog --check all pass.

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📚 Documentation preview 📚: https://sqlite-utils--898.org.readthedocs.build/en/898/