feat: full SQLAlchemy 2.0 support with 1.4 backward compat

[nwoolmer]

Summary

SQLAlchemy 2.0 compatibility while maintaining backward compat with SA 1.4. Bumps version to 1.2.0.

Dialect (dialect.py)

• Add import_dbapi() classmethod alongside dbapi() (fixes SADeprecationWarning - Implement an import_dbapi() #9, SQLAlchemy SADeprecationWarning #31, supersedes Fix SADeprecationWarning #33)
• Version-conditional create_engine params — future and implicit_returning only passed on SA <2.0 (removed in 2.0)
• SA version detected at import time via packaging.version.Version

Inspector (inspector.py)

• Engine.execute() is removed in SA 2.0 — all self.bind.execute() calls now go through a _get_connection() helper that opens a connection from the engine when needed, with proper cleanup

Packaging (pyproject.toml)

• Declare runtime dependencies: SQLAlchemy>=1.4, psycopg2-binary>=2.9, packaging
• Remove SQLAlchemy<2 cap from test dependencies
• Add Python 3.12/3.13 classifiers
• Add ruff ignores for new test file

Docker (Dockerfile, Makefile)

• Debian Buster is EOL (repos 404) — update to python:3.13-slim-bookworm
• Collapse RUN layers, remove stale SQLALCHEMY_SILENCE_UBER_WARNING
• pip install . now picks up declared runtime deps

Tests (test_sa2_compat.py)

• import_dbapi() returns the module
• dbapi() still works (backward compat)
• create_engine / create_superset_engine produce no deprecation warnings
• SA_V2 flag matches installed SA version

Backward compatibility

On SA 1.4 (as used by Superset 4.x/5.x which pins sqlalchemy>=1.4, <2), every code path produces identical behavior to the original:

• SA_V2 = False → future= and implicit_returning= are passed as before
• import_dbapi() exists but SA 1.4 calls dbapi() — both work
• Inspector's _get_connection() calls Engine.connect() which works on both SA versions

Test plan

• 28/28 tests pass on SQLAlchemy 2.0.48 + Python 3.13
• ruff check clean
• Docker image builds successfully
• Verify tests pass on SQLAlchemy 1.4.x (CI)

Closes #9, closes #31
Refs #35

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