Use sqlean if available in environment (#560)

Closes #559
Closes #235

Refs https://github.com/simonw/llm/issues/60

- Uses `sqlean` in place of `sqlite3` if `sqlean.py` is installed
- Uses `sqlite-dump` if available and `conn.iterdump()` does not exist
- New `with db.ensure_autocommit_off()` method for ensuring autocommit is off, used by `enable_wal()` and `disable_wal()`.

docs/installation.rst

@@ -39,6 +39,32 @@ Using pipx
 
     pipx install sqlite-utils
 
+.. _installation_sqlite3_alternatives:
+
+Alternatives to sqlite3
+=======================
+
+By default, ``sqlite-utils`` uses the ``sqlite3`` package bundled with the Python standard library.
+
+Depending on your operating system, this may come with some limitations.
+
+On some platforms the ability to load additional extensions (via ``conn.load_extension(...)`` or ``--load-extension=/path/to/extension``) may be disabled.
+
+You may also see the error ``sqlite3.OperationalError: table sqlite_master may not be modified`` when trying to alter an existing table.
+
+You can work around these limitations by installing either the `pysqlite3 <https://pypi.org/project/pysqlite3/>`__ package or the `sqlean.py <https://pypi.org/project/sqlean.py/>`__ package, both of which provide drop-in replacements for the standard library ``sqlite3`` module but with a recent version of SQLite and full support for loading extensions.
+
+To install ``sqlean.py`` (which has compiled binary wheels available for all major platforms) run the following:
+
+.. code-block:: bash
+
+    sqlite-utils install sqlean.py
+
+``pysqlite3`` and ``sqlean.py`` do not provide implementations of the ``.iterdump()`` method. To use that method (see :ref:`python_api_itedump`) or the ``sqlite-utils dump`` command you should also install the ``sqlite-dump`` package:
+
+.. code-block:: bash
+
+    sqlite-utils install sqlite-dump
 
 .. _installation_completion:
 

docs/python-api.rst

@@ -1779,6 +1779,25 @@ The ``db.sqlite_version`` property returns a tuple of integers representing the
     >>> db.sqlite_version
     (3, 36, 0)
 
+.. _python_api_itedump:
+
+Dumping the database to SQL
+===========================
+
+The ``db.iterdump()`` method returns a sequence of SQL strings representing a complete dump of the database. Use it like this:
+
+.. code-block:: python
+
+    full_sql = "".join(db.iterdump())
+
+This uses the `sqlite3.Connection.iterdump() <https://docs.python.org/3/library/sqlite3.html#sqlite3.Connection.iterdump>`__ method.
+
+If you are using ``pysqlite3`` or ``sqlean.py`` the underlying method may be missing. If you install the `sqlite-dump <https://pypi.org/project/sqlite-dump/>`__ package then the ``db.iterdump()`` method will use that implementation instead:
+
+.. code-block:: bash
+
+    pip install sqlite-dump
+
 .. _python_api_introspection:
 
 Introspecting tables and views