Use tomlkit to dump updated dependencies

[CasperWA]

Closes #203

Add a new utility function to update the parsed pyproject.toml file in-memory document, which will eventually be dumped (using tomlkit) and written to (overwriting) the pyproject.toml file.
This let's tomlkit handle the serialization (converting the parsed file back to TOML-syntax) and should be more stable than the current implementation using regular expressions.

As a side note, this now only writes to/updates the pyproject.toml file once - at the end of going through all dependencies if no errors occurred.
This is an (improved) change from the existing behaviour, which writes to/updates the pyproject.toml file for each dependency (if necessary) and does not revert changes if an error occurs. The improvement I see here, is that the pyproject.toml file now is only changed if there were no errors, plus the disk is touched only once for writing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.36%. Comparing base (cdd0d97) to head (212187e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
- Coverage   85.37%   85.36%   -0.02%     
==========================================
  Files          12       12              
  Lines         937      943       +6     
==========================================
+ Hits          800      805       +5     
- Misses        137      138       +1     

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[ajeklund]

I like this update! Seems like there is a problem with the Windows builds, though.

Partly out of scope-question regarding validation: Does the code check if there are multiple occurrences of a dependency where there should not be? Or will such user error be caught somewhere else?