Switch to tomli/tomllib for parsing TOML

[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

Replace the library used for parsing pyproject.toml by tomli on Python < 3.11 and tomllib on Python 3.11+.

tomli provides the same API as tomllib, which will be included in Python 3.11, so it is more future-proof.
Besides, toml is mostly unmaintained.

[fpgmaas]

Thanks for your contribution, much appreciated! This makes a lot of sense.

[codecov-commenter]

Codecov Report

Merging #123 (775156a) into main (44ea3d6) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #123   +/-   ##
=====================================
  Coverage   94.9%   95.0%           
=====================================
  Files         31      32    +1     
  Lines       1257    1260    +3     
=====================================
+ Hits        1194    1197    +3     
  Misses        63      63           

Impacted Files	Coverage Δ
tests/test_utils.py	100.0% <100.0%> (ø)

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