Use SingletonThreadPool for in-memory SQLite

Use SingletonThreadPool for in-memory SQLite
database created using filename uri

Default to using ``SingletonThreadPool`` for in-memory SQLite databases
created using URI filenames. Previously the default pool used was the
``NullPool`` that precented sharing the same database between multiple
engines.

Fixes: #6379
Closes: #6380
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6380
Pull-request-sha: 3b8024417a3a54c8269d43e40801aa4e58593540

Change-Id: Ice09622796455e796ede7711c98f3ceec13aa949