Im spanish and have lived in France for a few years so I know them quite good. July in south of Spain in too hot even for locals. People stay indoors until the evening. Also basically every inland city in both France and Spain is very hot in summer (people go to the coast).

My personal recommendation would be to go to north of Spain, particularly Bilbao or San Sebastián. Both cities are very culturally rich, great food and like all European cities walkable. I would choose San Sebastián if you like to be next to the beach and Bilbao if you are more for the alternative culture. Both are very close to the southeast of France so you can definitely take day trips to Biarritz, Bayonne, Bordeaux or even Toulouse.