Fix sync/async issues with Asgiref and officialize PsycopgConnector as the default one

[ewjoachim]

Closes #751

See #751 (comment)

Breaking changes:

• Aiopg and Psycopg2 connectors are now contrib. To use them, you'll need to import them with from procrastinate.contrib.aiopg import AiopgConnector and from procrastinate.contrib.psycopg2 import Psycopg2Connector. Also, the dependencies to aiopg and psycopg2 are now optional: use pip install procrastinate[aiopg] or pip install procrastinate[psycopg2] (or pip install procrastinate[aiopg,psycopg2] for both)
• The main supported connector is now the PsycopgConnector that uses Psycopg 3 (and SyncPsycopgConnector). Please note that PsycopgConnector accepts parameters based on psycopg_pool.ConnectionPool, which has a slightly different signature from the psycopg2/aiopg counterpart, the main difference is that the connection arguments are now passed as PsycopgConnector(kwargs={"host": "..."}) (instead of AiopgConnector(host=...)). Other parameters may have changed too, please check the documentation.
• Actually, you probably won't need sync connectors anymore because the Async connectors are now able to derive a sync connector when called in a sync context. You should try defining a single (async) connector in your code such as PsycopgConnector (or AiopgConnector), see if it works with all your existing code and don't hesitate to report potential issues. Synchronous connectors are still available in case you need it, so it should be a workaround in most cases.
• Synchronous tasks are now launched asynchronously in a ThreadPoolExecutor using asgiref.sync_to_async. That said, because of the Global Interpreter Lock (GIL), CPU-consuming tasks will not run faster with parallelization.
• The CLI parser has changed. The main differences should be around the order of arguments vs flag, and around environment variables. If you find something unexpected, please open an issue.
• Opening your app at the same time you instantiate it is now discouraged. Ideally, define your app as a module variable, and open it in the appropriate function (when your process starts). In the future, we may expose new helpers for doing this easily with django. Note that when you use the Procrastinate CLI, it takes care of opening/closing the app properly.
• JobManager.check_connection was an async method. It became check_connection_async for consistency. check_connection was created a the sync counterpart.

Successful PR Checklist:

• Tests
  • (not applicable?)
• Documentation
  • (not applicable?)

Todolist

• Launch acceptance tests with aiopg
• Put aiopg & psycopg2 dependencies as extras, update doc
• Implement get_sync_connector for aiopg (> psycopg2) and psycopg2 (>self)
• Maybe move aiopg & psycopg2 in contrib, update doc because we won't be able to import them from procrastinate directly. Add wrappers to make the error explicit
• Test the demo & quickstart extensively, redo demos documentation
• Investigate all the missing coverage spots & add corresponding tests
• Draft the exact list of methods that have been deleted for the breaking change message > Looks like no method was deleted :o
• Isolate asgiref dependency to utils.py
• Craft acceptable commits 😅

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate
__init__.py
__main__.py
app.py					264
cli.py					51
connector.py
exceptions.py
manager.py
psycopg_connector.py					164-166, 239, 298
schema.py
shell.py					41-45
sync_psycopg_connector.py					33, 151, 176
testing.py
utils.py
worker.py
procrastinate/contrib/aiopg
__init__.py
aiopg_connector.py					207-208
procrastinate/contrib/psycopg2
__init__.py
psycopg2_connector.py
procrastinate/contrib/sqlalchemy
psycopg2_connector.py					120
Project Total

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