Improve coverage report

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Coverage is currently set up to run statement coverage rather than branch one. Branch coverage is more granular, since it ensures that every possible branch of a function is measured.

This PR switches to branch coverage, and set a few more options:

• source specifies the files to measure (without that, tests themselves are currently measures in the report, which doesn't make sense)
• skip_empty removes empty files from the report

Additionally, let's take the occasion to bump pytest-cov to ^4.0.0 (changelog).

[codecov-commenter]

Codecov Report

Merging #153 (f9d5d3b) into main (1b12183) will increase coverage by 1.6%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #153     +/-   ##
=======================================
+ Coverage   92.9%   94.5%   +1.6%     
=======================================
  Files         43      25     -18     
  Lines       1355     804    -551     
  Branches       0     126    +126     
=======================================
- Hits        1259     760    -499     
+ Misses        96      29     -67     
- Partials       0      15     +15     

Impacted Files	Coverage Δ
deptry/compat.py	57.1% <0.0%> (-14.3%)	⬇️
deptry/utils.py	90.9% <0.0%> (-4.6%)	⬇️
deptry/issue_finders/transitive.py	92.0% <0.0%> (-4.0%)	⬇️
deptry/dependency_getter/requirements_txt.py	93.1% <0.0%> (-3.5%)	⬇️
deptry/issue_finders/obsolete.py	94.4% <0.0%> (-2.8%)	⬇️
deptry/dependency.py	91.2% <0.0%> (-1.8%)	⬇️
deptry/module.py	96.0% <0.0%> (-1.4%)	⬇️
deptry/import_parser.py	95.6% <0.0%> (-1.1%)	⬇️
tests/test_notebook_import_extractor.py
tests/dependency_getter/test_requirements_txt.py
... and 20 more

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[fpgmaas]

That looks good! I was not aware of the difference between statement and branch coverage. Learned something new again :)