build: make the database drivers optional extras

[dimitri-yatsenko]

Closes #1626.

Summary

pymysql is no longer a core dependency. Each database driver is an optional extra, and import datajoint loads no driver:

Extra	Installs
datajoint[postgres]	psycopg2-binary>=2.9.0 (unchanged)
datajoint[mysql]	pymysql>=0.7.2 (new; same floor as the former core dependency)

A bare pip install datajoint installs no database driver.

Changes

• pyproject.toml. pymysql moves from [project].dependencies to the new mysql extra. The test dependency group and the test and dev extras list both drivers.
• Lazy driver import. A new helper, import_driver in adapters/base.py, imports a driver when its adapter is created, which happens on connect. A missing driver raises a DataJointError that names the extra:

  The MySQL backend requires the pymysql driver, which is not installed. Install it with: pip install "datajoint[mysql]"
  

  adapters/mysql.py no longer imports pymysql at module load. adapters/postgres.py drops its module-level try: import psycopg2 (and the unused psycopg2.sql import). Its adapter raised ImportError for a missing driver and now raises the same DataJointError.
• psycopg2 vs psycopg2-binary. The adapter imports the psycopg2 module, which both distributions provide, so either one satisfies it.
• Other places that assumed pymysql is installed.
  • README install line: now shows datajoint[postgres] and datajoint[mysql].
  • CONTRIBUTING.md: the backend section lists both extras.
  • Development Dockerfile: installs /main[postgres,mysql].
  • RELEASE_MEMO.md: conda has no extras, so the feedstock recipe lists both drivers under run.
  • The lazy-import comment in __init__.py.

Migration

• MySQL: pip install "datajoint[mysql]", or datajoint[mysql] in a requirements file.
• PostgreSQL: pip install "datajoint[postgres]", as before.
• An upgrade in place keeps a pymysql that is already installed. A fresh environment built from a requirements file that lists only datajoint has no driver, and the first connection names the extra to install.
• Code that caught ImportError from PostgreSQLAdapter() should catch DataJointError.

Tests

• tests/unit/test_lazy_imports.py: in a fresh interpreter, import datajoint loads neither pymysql nor psycopg2.
• tests/unit/test_adapters.py::TestMissingDriver: blocks each driver in sys.modules, then checks two things:
  • get_adapter and Connection(...) raise a DataJointError naming the right extra, before any server is contacted.
  • Each backend needs only its own driver.
• Full suite (testcontainers, local): 1166 passed, 14 skipped.
• Clean virtual environments, each with an editable install of this branch:
  • Bare install: no driver installed. import datajoint succeeds with no driver loaded. dj.conn() raises ... pip install "datajoint[mysql]" (or [postgres] with DJ_BACKEND=postgresql).
  • [mysql]: installs PyMySQL only. dj.conn() reaches the MySQL driver, and psycopg2 is never imported.
  • [postgres]: installs psycopg2-binary only. dj.conn() reaches psycopg2, and pymysql is never imported.

Notes for review

• pixi.lock is not regenerated in this PR. Re-solving it changes no package versions in the existing default, dev, and test environments. The only changes are that default drops pymysql and gains packaging, which the committed lock was already missing. The rest of the re-solve adds the test-py310/test-py314 environments, which the open Python 3.15 build PR (build: support Python 3.15 #1596) also adds. pixi.lock merges as binary, so regenerating it in both PRs would conflict. CI runs with locked: false.
• datajoint[mysql] installs plain pymysql. Without cryptography, pymysql cannot complete caching_sha2_password authentication over a non-TLS connection. This was equally true when pymysql was a core dependency. pymysql[rsa] would add cryptography.