scheduling: fix scheduling times in UTC timezone instead of local time

[maurerle]

When using the RecurrentScheduledTime, I would expect to have the time interpreted in UTC.

Conversion between timestamp and datetime is generally depending on the host timezone.

For example:

>>> datetime.fromtimestamp(0, tz=timezone.utc).replace(tzinfo=None)
datetime.datetime(1970, 1, 1, 0, 0)
>>> datetime.fromtimestamp(0)
datetime.datetime(1970, 1, 1, 1, 0)

>>> datetime(1970,1,1).timestamp()
-3600.0
>>> import calendar
>>> calendar.timegm(datetime(1970,1,1).utctimetuple())
0

For a simulation tool, this is rather not expected as this gives different results depending on the host location, so I would rather expect it to use UTC wherever possible.
This is done in this PR, though remaining occurances in test functions of datetime.timestamp() are still present.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.19%. Comparing base (3695e9f) to head (bffc1a2).
Report is 8 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #160      +/-   ##
===============================================
+ Coverage        89.11%   89.19%   +0.07%     
===============================================
  Files               22       22              
  Lines             2490     2498       +8     
===============================================
+ Hits              2219     2228       +9     
+ Misses             271      270       -1     

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