Update in-memory database handling

[dspinellis]

When rogerbinns/apsw#553 is resolved:

• Unpin apsw version from Pipfile.
• Use the new documented URI format in the "ATTACH DATABASE 'file:virtual?mode=memory&cache=shared' AS virtual" and self.vdb = apsw.Connection("file:virtual?mode=memory&cache=shared", apsw.SQLITE_OPEN_URI | apsw.SQLITE_OPEN_READWRITE) statements.