[1.8.x] Allowed using the sqlite3 backend as much as possible without _sqlite3.

The inner import was dropped during the reorganizing of the database
backends in commit 28308078f3.

Backport of 95058606c8 from master

django/db/backends/sqlite3/schema.py

@@ -1,4 +1,3 @@
-import _sqlite3  # isort:skip
 import codecs
 import copy
 from decimal import Decimal
@@ -14,6 +13,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_create_inline_fk = "REFERENCES %(to_table)s (%(to_column)s)"
 
     def quote_value(self, value):
+        # The backend "mostly works" without this function and there are use
+        # cases for compiling Python without the sqlite3 libraries (e.g.
+        # security hardening).
+        import _sqlite3
         try:
             value = _sqlite3.adapt(value)
         except _sqlite3.ProgrammingError: