Update `flight.py`

[Dhud6]

Description

Add four new data members:

• expected_departure_time: int
• expected_arrival_time: int
• actual_departure_time: int
• actual_arrival_time: int

The expected departure and arrival times should be added to the constructor since they can be derived from the expected travel time of the route, boarding, and deboarding times. The actual times must be initialized after the flight has landed, you you should assign then None

Testing

Also add unit tests

[Dhud6]

Updated flight.py to add expected_departure_time, expected_arrival_time, actual_departure_time, and actual_arrival_time to the constructor. Need to add the unit tests via pytest.

[Dhud6]

Created the initial unit test setup with test_flight.py. Still needs unit tests for value and type checking.

[Dhud6]

Finished adding tests to the test_flight.py. Also added unit tests for test_airport.py.