Fix ninjotiff writer for mode P

[mraspaud]

This PR ensures that images with mode P are in uint8 dtype when sent to pyninjotiff

• Tests added

[codecov]

Codecov Report

Merging #1817 (75ddb77) into main (4c475fa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1817   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         266      266           
  Lines       39243    39257   +14     
=======================================
+ Hits        36517    36531   +14     
  Misses       2726     2726           

Flag	Coverage Δ
behaviourtests	4.74% <0.00%> (-0.01%)	⬇️
unittests	93.59% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_ninjotiff.py	100.00% <100.00%> (ø)
satpy/writers/ninjotiff.py	79.66% <100.00%> (+0.71%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c475fa...75ddb77. Read the comment docs.

[coveralls]

Coverage increased (+0.002%) to 93.535% when pulling 75ddb77 on mraspaud:fix-ninjo-p into 4c475fa on pytroll:main.