Tweak coverage exclusions

[mthuurne]

By excluding things we don't intend to cover, lines that should be covered but are not stand out more.

[codecov]

Codecov Report

Merging #281 (8ff0c6b) into master (13749a4) will increase coverage by 1.84%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
+ Coverage   81.89%   83.74%   +1.84%     
==========================================
  Files          25       23       -2     
  Lines        3812     3728      -84     
  Branches      792      773      -19     
==========================================
  Hits         3122     3122              
+ Misses        476      402      -74     
+ Partials      214      204      -10     

Impacted Files	Coverage Δ
pydoctor/sphinx_ext/build_apidocs.py
pydoctor/sphinx_ext/_help_output.py
pydoctor/epydoc/markup/restructuredtext.py	81.44% <0.00%> (+0.36%)	⬆️
pydoctor/driver.py	62.50% <0.00%> (+0.61%)	⬆️
pydoctor/model.py	88.62% <0.00%> (+0.67%)	⬆️
pydoctor/epydoc/markup/epytext.py	77.49% <0.00%> (+1.90%)	⬆️
pydoctor/epydoc/doctest.py	100.00% <0.00%> (+2.35%)	⬆️
pydoctor/sphinx.py	98.36% <0.00%> (+2.61%)	⬆️
pydoctor/epydoc/markup/__init__.py	86.20% <0.00%> (+7.41%)	⬆️

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[adiroiban]

Thanks. All good.