There are two problems with averages: One is that they hide extremes, and it's the extremes that are miserable; the other is that there are averages floating around out there (such as on cities' Wikipedia pages) that are based on timeframes that cut off in the year 2010. It's my impression that weather has gotten considerably more erratic since then, so I don't think those averages are necessarily very accurate.
I prefer to look at actual day-by-day weather data for my proposed travel period over the most recent five years or more. That information is usually available from timeanddate.com. I've linked to the August 2018 stats for Avignon (destination chosen randomly). Do check multiple years.
One other point: If this is your first summer trip to Europe, you may not realize how comparatively under-air-conditioned Europe is, or how much being a traveler in Europe differs from your normal life at home. At home most of us move from our air-conditioned homes to our air conditioned cars to an air-conditioned work place, store, etc. Unless we are employed as gardeners or golf-course groundskeepers, we are not moving around outdoors in the heat for large chunks of the day. It really is a completely different situation when we play tourist on a trip to Europe.
It will help if you are traveling by (air-conditioned) rental car rather than depending on public transportation, but you'll still encounter many museums, cafes, restaurants and shops without air conditioning. Note: It's not that buses and trains aren't air-conditioned; they usually are. It's the frequent lack of air conditioning where you wait to board the bus or train.
It is a very good idea to try to select lodgings that are close to the tourist center of town so you can take a mid-day break in your air-conditioned room if you need to. Unfortunately, that won't help when you're on a day-trip; that's where the air-conditioned car comes in.
We joke sometimes about looking for a supermarket so we can hang out in the frozen-food section to cool off. Unfortunately, it's not really a joke.
This public-service message brought to you by someone who spent mid-June to mid-July 2015 in Sicily and Puglia. Obviously, I lived to tell you about the experience, but that was really poor planning on my part, and that portion of the trip would have been a lot more pleasant earlier or later in the year. A few days of extreme heat is something you just gut out with the help of lots of cold water; it gets tougher the longer you're exposed to it.
If you decide to proceed, I suggest you look at a product called a "Kool Tie", which I find somewhat helpful, though I'm not going to claim you won't know it's hot when you use it. There seem to be similar products out there, but this is the one I have used.
