fix: work with recent apsw

It looks like from apsw 3.43.1.1 onwards, the virtual filesystem file needs the
xSectorSize function.

Nothing in the apsw changelog at https://rogerbinns.github.io/apsw/changes.html
makes this particular clear - the issue from 3.43.1.1 onwards was determined by
running the tests for sqlite-s3vfs for each apsw version.

Thank you to https://github.com/blueshed for the report and the fix.

Closes #24

sqlite_s3vfs.py

@@ -128,6 +128,9 @@ def _read():
 
         return b"".join(_read())
 
+    def xSectorSize(self):
+        return self._block_size
+
     def xFileControl(self, *args):
         return False