feat: add psycopg3 support

[danfimov]

closes #17

[Copilot]

Pull Request Overview

This pull request adds support for psycopg3 as a PostgreSQL driver alongside the existing asyncpg, psqlpy, and aiopg drivers. The implementation includes a complete broker, result backend, and schedule source for psycopg3, along with comprehensive integration tests.

Key Changes:

• Implementation of PsycopgBroker, PsycopgResultBackend, and PsycopgScheduleSource
• Refactored integration tests to use fixtures and reduce duplication
• Updated existing broker implementations to use private attributes for connections
• Added database cleanup fixtures for better test isolation

Reviewed Changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/taskiq_pg/psycopg/*.py	New psycopg3 implementation files (broker, result backend, schedule source, queries, init)
src/taskiq_pg/psqlpy/broker.py	Refactored to use private attributes and removed redundant null checks
src/taskiq_pg/psqlpy/schedule_source.py	Updated to use JSONB type wrapper for schedule data
src/taskiq_pg/asyncpg/broker.py	Refactored to use private attributes and added listener cleanup
tests/integration/test_broker.py	New consolidated broker tests with fixtures
tests/integration/test_schedule_source.py	Refactored tests with fixtures and added psycopg support
tests/integration/test_result_backend.py	Added psycopg to parameterized tests
tests/integration/conftest.py	Added database cleanup and connection fixtures
pyproject.toml	Added psycopg dependency and sqlalchemy-utils for tests
docs/index.md, README.md	Updated documentation with psycopg examples

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