Add Meteosat-12 to platforms.txt

[gerritholl]

• Closes pyorbital cannot read TLE for MTG-I1 / Meteosat-12 #140
• Tests added
• Tests passed
• Passes flake8 pyorbital

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e76264b) 87.51% compared to head (15e6bb6) 87.51%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #141   +/-   ##
=======================================
  Coverage   87.51%   87.51%           
=======================================
  Files          14       14           
  Lines        2210     2210           
=======================================
  Hits         1934     1934           
  Misses        276      276           

Flag	Coverage Δ
unittests	87.51% <ø> (ø)

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[coveralls]

coverage: 87.562%. remained the same
when pulling 15e6bb6 on gerritholl:meteosat12
into e76264b on pytroll:main.

[gerritholl]

Does this need a unit test?

[djhoese]

I vote for no tests needed unless there are already some equivalent/similar tests. I can merge this if you're OK with that @gerritholl.