text: Title: Update `dbapi` method to `import_dbapi` for `APSWGSheetsDialect` to remove warning at the start of program
  Body: **Is your feature request related to a problem? Please describe.**
  For newer versions of SQLAlchemy (2.0), usage of `dbapi()` class method is deprecated. It is recommended to implement `import_dbapi()` class method. [Docs](https://docs.sqlalchemy.org/en/20/core/internals.html#sqlalchemy.engine.Dialect.import_dbapi)
  So, the following warning is shown every time I use SQLAlchemy with Google Sheets adapter:
  ```
  SADeprecationWarning: The dbapi() class method on dialect classes has been renamed to import_dbapi().  Implement an import_dbapi() class method directly on class <class 'shillelagh.backends.apsw.dialects.gsheets.APSWGSheetsDialect'> to remove this warning; the old .dbapi() class method may be maintained for backward compatibility.
  ```
  
  **Describe the solution you'd like**
  I would like to update the base class `APSWDialect`. Remove (if it does not break anything, I am not sure how it really works rn) or extend the class with the class method `import_dbapi`.
  
  **Describe alternatives you've considered**
  Leaving as it is for now. But as it is deprecated, better follow the docs.
  
  **Additional context**
  Python 3.11
  SQLAlchemy==2.0.20
  importlib-metadata==6.8.0 (without it SQLAlchemy, ORM to be exact, does not work)
  