Remove DataID attribute access in seviri_l2_grib reader

[strandgren]

The recently merged PR #2396, removed the DataID attribute access and modified affected readers accordingly. However, two instances in the seviri_l2_grib reader were not modified leading to failure when using this reader with the current main. This PR resolves this issue.

The initial commit fixed the attribute access, but broke the reader's unit test since a mocked DataID was used, which lead to TypeError since the mock object was not subscriptable. This was resolved by using make_dataid for the test instead.

[djhoese]

The initial commit fixed the attribute access, but broke the reader's unit test since a mocked DataID was used, which lead to TypeError since the mock object was not subscriptable. This was resolved by using make_dataid for the test instead.

Over-mocking in tests gets us again.

[djhoese]

If there is any type of urgency to this PR for your/EUM's work, I'm OK merging it into main if you can confirm that all tests pass locally. We'll deal with the NetCDF test failure in CI in another PR.

[strandgren]

@djhoese I'm missing packages needed to run all the tests, but I can confirm that test_seviri_l2_grib passes locally. But unless we're talking weeks, it's not that urgent.

[mraspaud]

I just merged #2412 , so the tests should be fixed now if you merge main into this PR

[codecov]

Codecov Report

Merging #2408 (11c28a4) into main (673d2e9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2408   +/-   ##
=======================================
  Coverage   94.76%   94.76%           
=======================================
  Files         329      329           
  Lines       48878    48879    +1     
=======================================
+ Hits        46318    46319    +1     
  Misses       2560     2560           

Flag	Coverage Δ
behaviourtests	4.42% <0.00%> (-0.01%)	⬇️
unittests	95.39% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/readers/seviri_l2_grib.py	92.47% <100.00%> (ø)
satpy/tests/reader_tests/test_seviri_l2_grib.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage: 95.349% (+0.0001%) from 95.349% when pulling 11c28a4 on strandgren:bugfix_seviri_grib_attribute_access into 673d2e9 on pytroll:main.