Add NOAA-21 support

[adybbroe]

This PR is adding NOAA-21 to the list of designators/ids in the built in platforms.txt file

This was actually already included in #113 but that PR is about environment variables and is also not yet ready and merged.

• [ ] Closes #xxxx
• [x] Tests added
• [ ] Fully documented

[codecov[bot]]

Codecov Report

Merging #118 (e828412) into main (8d982a9) will increase coverage by 0.29%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   87.51%   87.81%   +0.29%     
==========================================
  Files          15       15              
  Lines        2211     2248      +37     
==========================================
+ Hits         1935     1974      +39     
+ Misses        276      274       -2     

Flag	Coverage Δ
unittests	87.81% <100.00%> (+0.29%)	:arrow_up:

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

Impacted Files	Coverage Δ
pyorbital/tests/test_tlefile.py	99.77% <100.00%> (+0.02%)	:arrow_up:
pyorbital/tlefile.py	93.66% <100.00%> (+0.54%)	:arrow_up:

[adybbroe]

While working with this PR and upgrading our systems for NOAA-21 at SMHI we realized that there seem to be a bug here, standing in the way of updating support for NOAA-21, see #119 So, perhaps this PR should try fix this as well?