Ethyl 4-hydroxy-2,6-diphenyl-5-(phenylsulfanyl)pyridine-3-carboxylate and ethyl 2,6-bis(4-fluorophenyl)-4-hydroxy-5-(4-methylphenylsulfanyl)pyridine-3-carboxylate: supramolecular aggregation through C—H...O, C—H...F and C—H...π interactions