fix: update code to dependency updates

[pierrecamilleri]

CI was failing because several dependencies (with no major version pin) have introduced breaking changes or changed behavior.

• chardet v7 has different character encoding detections that chardet v5. Several tests were broken.
  • some tests were simply testing the (wrong) detection from chardet. They are now testing the correct/most likely encoding of the file (UTF-8).
  • test files are really short, with a unique non-ascii character. Chardet's confidence is therefore very low, and it cannot distinguish several encodings. Some tests were made less strict ("Check that the non-ascii is correctly decoded" instead of "check the exact encoding"), and the sample size to detect encoding has been augmented (x10).
• there was a dependency on pytz, which was a (former) transitive dependency (from pandas maybe ?). It has been replaced with datetime.
• Pandas v2 to v3 changes its dtypes
  • adapt to new dtypes collection type
  • make the tests accept former and new dtypes alongside
  • deal with new "nan" handling
• adapt to changes of pytest-cov v7.
• pin TatSu < 5.15
• pin virtualenv < 21 for hatch (hatch issue, still applies to 3.8 and 3.9)

Additionnaly :

• add CI tests for python 3.13 and 3.14
• depreciation warning for python 3.8 and 3.9
• CI for py3.8 takes ages, so I removed it.