Revert "Use member variable `self.conn` to avoid test failure"
Revert "Use member variable `self.conn` to avoid test failure"

This reverts commit 4fafdfd8487ddf61105b2a5ca92534c8014c1db4.

While using `self.conn.cursor()` works in the tests, it will not work
in the application since the point of initialisation could be very
different from the point where the 'property' is called.

genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2