Create a new `Vet` class for human`

[khider]

In the human.py file, create a new Vet class to simulate a veterinarian.

• Add the class to human.py with the following parameters: clinic_name which takes a string and defaults to None and patients which takes a list of Cats and defaults to None. Make sure that if patients is a list then the list contains Cats
• Write a function called checkup that takes a Cat as a parameter and increase cat health by 1 and decreases its mode by 1
• Write docstrings
• Write tests for the class and functions.