Fix mypy errors in course.py

[tianyizheng02]

Run mypy with --strict flag and fix resulting mypy errors in course.py

Contributes to #45 by fixing existing type hint errors prior to introducing mypy to the repo.

Before:

> mypy --strict pittapi/course.py
pittapi/course.py:251: error: Argument 2 to "_get_section_details" has incompatible type "str | int"; expected "str"  [arg-type]
pittapi/course.py:361: error: Missing type parameters for generic type "dict"  [type-arg]
pittapi/course.py:362: error: Returning Any from function declared to return "dict[Any, Any]"  [no-any-return]
pittapi/course.py:365: error: Missing type parameters for generic type "dict"  [type-arg]
pittapi/course.py:366: error: Returning Any from function declared to return "dict[Any, Any]"  [no-any-return]
pittapi/course.py:369: error: Missing type parameters for generic type "dict"  [type-arg]
pittapi/course.py:373: error: Returning Any from function declared to return "dict[Any, Any]"  [no-any-return]
pittapi/course.py:376: error: Missing type parameters for generic type "dict"  [type-arg]
pittapi/course.py:380: error: Returning Any from function declared to return "dict[Any, Any]"  [no-any-return]
pittapi/course.py:383: error: Missing type parameters for generic type "dict"  [type-arg]
pittapi/course.py:387: error: Returning Any from function declared to return "dict[Any, Any]"  [no-any-return]
pittapi/course.py:399: error: Missing type parameters for generic type "dict"  [type-arg]
pittapi/course.py:412: error: Returning Any from function declared to return "str"  [no-any-return]
Found 13 errors in 1 file (checked 1 source file)

After:

> mypy --strict pittapi/course.py 
Success: no issues found in 1 source file