fix: allow string_content to be a str

[BoboTiG]

I was surprised to read the events() docstring stating:

:param string_content: iCal content as string

... While not being able to pass a str. Here is the error in this case:

Traceback (most recent call last):
  File "test.py", line 28, in <module>
    events = icalevents.events(
        string_content=ics,
    ...<3 lines>...
        sort=True,
    )
  File "/.../icalevents/icalevents.py", line 55, in events
    content = ical_download.data_from_string(string_content, apple_fix=fix_apple)
  File "/.../icalevents/icaldownload.py", line 91, in data_from_string
    return self.decode(string_content, apple_fix=apple_fix)
           ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.../icalevents/icaldownload.py", line 103, in decode
    content = content.decode(encoding)
              ^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'decode'. Did you mean: 'encode'?

So here is a small patch to allow both bytes, and str.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.90%. Comparing base (5dd30fe) to head (0659836).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
+ Coverage   91.88%   91.90%   +0.02%     
==========================================
  Files           4        4              
  Lines         382      383       +1     
  Branches       93       94       +1     
==========================================
+ Hits          351      352       +1     
  Misses         14       14              
  Partials       17       17              

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