How can I crop a video?
Handbrake is the way to go here, as mentioned by roguesys and  canary_in_the_data_mine in the accepted answer. 

I am using Catalina. MPEG Streamclip does not support this version of OSX, and iMovie's cropping tool is married to the 16:9 aspect ratio. 

As soon as you open your clip, if it has black bars (which was my case), handbrake will crop them automatically and it will show you in its summary how the converted video will look like. 

You can tweak out things in the other tabs (dimensions where you can do a fine tune of the cropping, video for the compression codec and other things, filters, audio, etc). 

Once you're satisfied with the settings in the summary, go to the bottom of the App window, choose the folder where you will save the video, and press the green Start button at the top.