Rot13 app - How to tidy up code?
Your ROT algorithm is slightly wrong. The real ROT returns The alphabet rotated in the corresponding case. But in yours, even if you give Uppercase(TUVWXYZ), it returns lowercase for large rotations. That is not exact ROT.
This can be seen from this screenshot 
This may be helpful:  https://www.geeksforgeeks.org/rot13-cipher/. Here instead of chr() and ord() Dictionaries are used which is very efficient and less error prone.