Add type hints [beginner, Python, typing]

[niccokunzmann]

icalendar exists since Python 2. Nowadays, type hints are a huge support for developers!

If you are a newcomer to icalendar, please add type hints to untyped functions. Here is your checklist:

• choose a function that does not have a return type or argument types in src/icalendar
• find all places where it is used (easiest for functions that are not used much)
• from its content and usage, add type hints
• create a pull request with your changes, mentioning this issue.

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