[2.1.x] Fixed #29182 -- Fixed schema table alteration on SQLite 3.26+.

SQLite 3.26 repoints foreign key constraints on table renames even when
foreign_keys pragma is off which breaks every operation that requires
a table rebuild to simulate unsupported ALTER TABLE statements.

The newly introduced legacy_alter_table pragma disables this behavior
and restores the previous schema editor assumptions.

Thanks Florian Apolloner, Christoph Trassl, Chris Lamb for the report and
troubleshooting assistance.

Backport of c8ffdbe514 from master.

django/db/backends/sqlite3/schema.py

@@ -21,10 +21,12 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         # Some SQLite schema alterations need foreign key constraints to be
         # disabled. Enforce it here for the duration of the transaction.
         self.connection.disable_constraint_checking()
+        self.connection.cursor().execute('PRAGMA legacy_alter_table = ON')
         return super().__enter__()
 
     def __exit__(self, exc_type, exc_value, traceback):
         super().__exit__(exc_type, exc_value, traceback)
+        self.connection.cursor().execute('PRAGMA legacy_alter_table = OFF')
         self.connection.enable_constraint_checking()
 
     def quote_value(self, value):

docs/releases/2.0.10.txt

@@ -12,3 +12,7 @@ Bugfixes
 * Prevented repetitive calls to ``geos_version_tuple()`` in the ``WKBWriter``
   class in an attempt to fix a random crash involving ``LooseVersion`` since
   Django 2.0.6 (:ticket:`29959`).
+
+* Fixed a schema corruption issue on SQLite 3.26+. You might have to drop and
+  rebuild your SQLite database if you applied a migration while using an older
+  version of Django with SQLite 3.26 or later (:ticket:`29182`).

docs/releases/2.1.5.txt

@@ -11,3 +11,7 @@ Bugfixes
 ========
 
 * Fixed compatibility with mysqlclient 1.3.14 (:ticket:`30013`).
+
+* Fixed a schema corruption issue on SQLite 3.26+. You might have to drop and
+  rebuild your SQLite database if you applied a migration while using an older
+  version of Django with SQLite 3.26 or later (:ticket:`29182`).