Add new features

[ShadowMikado]

I have done all of the following:

• Added the feature.
• Added documentation for the feature.
• Added tests for the feature.
• Ran mypy, black, and unit tests.

DOCUMENTATION

I have added one new feature:

get_calendar: allows retrieving a unique URL for a PRONOTE user's calendar. Returns a unique url to access the user's calendar (I don't know if this is available for other PRONOTE systems edited: Working), combined with client.pronote_url to retrieve the full unique URL.

get_last_connection allows getting the last connection date, returns datetime.datetime