Allow to read calendars from files

[niccokunzmann]

In order to make it nicer to use the icalendar interface, we can add these two types to icalendar.Calendar.from_ical():

• pathlib.Path
• str

In both cases, we assume a path is given.

Implementation suggestion

• translate str into Path
• use Path.read_bytes() to get the content that would usually be passed to the from_ical()
• document all the parameters, see Which docstring format should we use? #747
• Test it with Path and with str

Context

Usually, these are .ics files. As such, the other components do not need that feature.

[abe-101]

I propose adding a new method to the Calendar object
icalendar.Calendar.from_file()

and the method will open the provided file and pass the content to
icalendar.Calendar.from_ical()

[abe-101]

I think we can do the same for icalendar.Calendar.to_ical()
a new method called icalendar.Calendar.to_file() that excepts a path and uses to_ical() and write it to the given path