Add get_observer_look test for scalar case and update stickler config

[gerritholl]

Add a get_observer_look unit test when passing scalars / regular Python types.

Although #90 was a false alarm and had already been fixed in #77, it appears this may have been accidental, as no mention of this bug is made in #77 or #72. Add a unit test for passing scalars to get_observer_look to make sure we cover this case in the future.

Also updates stickler configuration to make sure it uses Python 3 and respects configuration from setup.cfg.

• [x] Closes #91
• [x] Tests added
• [x] Tests passed
• [x] Passes flake8 pyorbital

[codecov[bot]]

Codecov Report

Merging #91 (104ba09) into main (cbe67e2) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   86.64%   86.67%   +0.03%     
==========================================
  Files          13       13              
  Lines        2021     2026       +5     
==========================================
+ Hits         1751     1756       +5     
  Misses        270      270              

Flag	Coverage Δ
unittests	86.67% <100.00%> (+0.03%)	:arrow_up:

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Impacted Files	Coverage Δ
pyorbital/tests/test_orbital.py	96.28% <100.00%> (+0.06%)	:arrow_up:
pyorbital/tlefile.py	91.06% <0.00%> (+0.02%)	:arrow_up:

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