Allow to pass pathlike-objects to FSFile

[gerritholl]

When constructing an FSFile object, allow to pass not only strings, but
any object meeting the os.PathLike protocol.

• Closes FSFile should support any PathLike objects #1516
• Tests added
• Passes flake8 satpy
• Fully documented

[codecov]

Codecov Report

Merging #1519 (3dcf240) into master (653afc0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1519   +/-   ##
=======================================
  Coverage   91.65%   91.65%           
=======================================
  Files         248      248           
  Lines       36167    36172    +5     
=======================================
+ Hits        33150    33155    +5     
  Misses       3017     3017           

Flag	Coverage Δ
behaviourtests	4.35% <0.00%> (-0.01%)	⬇️
unittests	92.19% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/readers/__init__.py	97.40% <100.00%> (ø)
satpy/tests/test_readers.py	99.20% <100.00%> (+<0.01%)	⬆️

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[gerritholl]

As often, the codecov report makes no sense. I've only added test code and test coverage has gone down?

[djhoese]

Thanks for fixing it. One small docstring change please.

[djhoese]

Thanks!

[mraspaud]

LGTM, thanks for adding this functionality