Use scene properties instead of .attrs where needed

[pnuu]

In Satpy the Scene object no longer have metadata in the .attrs dictionary. This PR replaces that usage by collecting the relevant items from the .start_time, .end_time and .sensor_names attributes/properties where required.

The required Satpy version is set to >=0.32.0 to be certain .sensor_names attribute is present.

• [x] Closes #136
• [x] Tests added
• [x] Tests passed
• [x] Passes flake8 trollflow2

[codecov[bot]]

Codecov Report

Merging #137 (0bc72aa) into main (d00d2e0) will increase coverage by 0.24%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   95.16%   95.40%   +0.24%     
==========================================
  Files          11       11              
  Lines        2296     2331      +35     
==========================================
+ Hits         2185     2224      +39     
+ Misses        111      107       -4     

Flag	Coverage Δ
unittests	95.40% <100.00%> (+0.24%)	:arrow_up:

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Impacted Files	Coverage Δ
trollflow2/plugins/__init__.py	92.80% <100.00%> (+0.84%)	:arrow_up:
trollflow2/tests/test_trollflow2.py	99.45% <100.00%> (+0.01%)	:arrow_up:

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[gerritholl]

Thanks for the quick work! We'll try it out. My only worry is that the unit tests still rely on mocking the interface with Satpy, such that next time the Satpy scene object changes its API, it won't be caught by those tests…