Fixed #3214 -- Stopped parsing SQL with regex.

Avoided introducing a new regex-based SQL splitter in the migrations
framework, before we're bound by backwards compatibility.

Adapted this change to the legacy "initial SQL data" feature, even
though it's already deprecated, in order to facilitate the transition
to migrations.

sqlparse becomes mandatory for RunSQL on some databases (all but
PostgreSQL). There's no API to provide a single statement and tell
Django not to attempt splitting. Since we have a more robust splitting
implementation, that seems like a good tradeoff. It's easier to add a
new keyword argument later if necessary than to remove one.

Many people contributed to both tickets, thank you all, and especially
Claude for the review.

Refs #22401.

docs/internals/contributing/writing-code/unit-tests.txt

@@ -166,6 +166,7 @@ dependencies:
 *  memcached_, plus a :ref:`supported Python binding <memcached>`
 *  gettext_ (:ref:`gettext_on_windows`)
 *  selenium_
+*  sqlparse_
 
 You can find these dependencies in `pip requirements files`_ inside the
 ``tests/requirements`` directory of the Django source tree and install them
@@ -197,6 +198,7 @@ associated tests will be skipped.
 .. _memcached: http://memcached.org/
 .. _gettext: http://www.gnu.org/software/gettext/manual/gettext.html
 .. _selenium: https://pypi.python.org/pypi/selenium
+.. _sqlparse: https://pypi.python.org/pypi/sqlparse
 .. _pip requirements files: http://www.pip-installer.org/en/latest/cookbook.html#requirements-files
 
 Code coverage