Is it too hot to visit Evora in July?

The temperatures in Évora in July are in the thirties (Celsius).
Since you're from Texas you're probably used to those temperatures and higher, but you might use A/C .
A/C in public places is gaining popularity but is not yet widespread - restaurants,etc.

You can also do what the locals did before A/C, active in the morning, relaxed after lunch with a mid day sesta (nap) and become active again later in the afternoon , drinking lots of fluids and not exerting themselves.

https://www.portugalresident.com/portugal-braces-for-heatwave-lasting-several-weeks/

In a word - YES! I would never go anyplace in the Alentejo in July. The biggest difference is that AC is not as plentiful as in the States. And, when there is AC, it’s usually not nearly as powerful so stays much warmer than you expect.

All that said, Evora is an easy day trip from Lisbon. So, you might luck out and have an unusually cool day while you’re here. You could then quickly change your plans and go for the day.

I am not telling you to change your travel style, but we happened to use a packaged local bus company tour for a day, which did Evora and a couple of other sites, returning to Lisbon in the early evening. We did this not because we're terrified of independent travel (we later rented a car, which we returned in Porto, I think.) It was just a way to see several key sights in a single day, with no navigation efforts, parking, and so on. It was a short walk in Lisbon from our hotel to the tour company.

One point of this is that the usual objections to these tours ("You get little free time at the location ... ", for example) means you know that you will be back on the air conditioned 49-passenger bus after a fixed interval of time, and off to the next location. I agree that Americans often hate being on someone else's schedule, (or worse, having someone else at the wheel!) but I'm just making a suggestion.

Our trip was so long ago that I'm not going to try to look up the company, but this seems like an area where (absent a personal recommendation) the bigger, the better. And there are online reviews today.

If July is when your trip is and you want to visit Evora, visit it. Yes it will be hot so plan your activities for the morning if possible. Have plenty of water. Depending on how hot, hot is, anticipate not seeing as many sights as you planned on and taking more breaks. Decades ago we were in that area in August and it was on that trip that we decided we’d never travel during the summer months again. We did have a rental car.

Yes. It is extremely hot in July.