Keep asyncpg installed by default in the Postgres provider

[shahar1]

Problem

Postgres provider 7.0.0 (currently in RC as 7.0.0rc1, #69526 / merged #69089) made psycopg3 the default async metadata-DB driver and, as part of that, moved asyncpg from a default dependency to the opt-in [asyncpg] extra.

The psycopg-async default and its postgresql+asyncpg:// fallback only exist on Airflow 3.4.0+ (airflow-core settings.py::_get_async_conn_uri_from_sync). Every currently released 3.x core (3.0–3.3), which this provider still supports (apache-airflow>=2.11.0), derives the async metadata-DB URL as postgresql+asyncpg:// unconditionally and has no psycopg fallback.

So upgrading the Postgres provider to 7.0.0 on any released 3.x core (or a 3.4.0 deployment that pinned an older core) leaves the async metadata-DB engine unable to load its driver:

sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgresql.asyncpg

This is a regression against cores the provider claims to support, and a reason to hold postgres 7.0.0rc1.

Fix

Keep asyncpg as a default dependency of the Postgres provider until its minimum supported Airflow is 3.4.0 (i.e. until every supported core knows the psycopg_async fallback). psycopg3 remains the default async driver on 3.4.0+; asyncpg is simply still present so older supported cores keep working. The removal is gated on the floor bump and noted at the workaround site.

• pyproject.toml: add asyncpg>=0.30.0 back to default dependencies (comment explains the constraint + tracking issue).
• changelog.rst: correct the 7.0.0 note — asyncpg stays installed by default; the psycopg_async switch is 3.4.0+ behavior only.
• README.rst / docs/index.rst / uv.lock: regenerated by prek hooks.

The eventual removal (converge on psycopg3-only) is tracked at #68453.

related: #69526
related: #68453

Test plan

Dependency/packaging + changelog change; no Python behavior to unit-test (the affected async-URL derivation lives in airflow-core, unchanged here). Verified: prek Update dependencies for providers, Sync provider README.rst Requirements table with pyproject.toml, Update uv.lock, and changelog-format hooks all pass.

Note: two related core hardening items remain as separate follow-ups (not required to unblock the RC): core _USE_PSYCOPG3 gates on find_spec("psycopg") only, missing the SQLAlchemy-2 guard the provider hook has; and the derived async URL is not validated against installed drivers, so failures surface as an opaque NoSuchModuleError.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.8)

Generated-by: Claude Code (Opus 4.8) following the guidelines

[Dev-iL]

Thank you for opening this. The reasoning is sound, the implementation looks correct.