Improve time stamp parsing performance

[soininen]

We now try parsing time stamps with datetime.fromisoformat() before falling back to dateutil.parser.parse() which makes e.g. parsing DateTime objects from database ~x10 faster.

Resolves #385

Checklist before merging

• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 20.00000% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 78.22%. Comparing base (ade116a) to head (b219ceb). Report is 2 commits behind head on 0.8-dev.

Files	Patch %	Lines
spinedb_api/parameter_value.py	20.00%	16 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #388 +/- ## =========================================== - Coverage 78.32% 78.22% -0.10% =========================================== Files 75 75 Lines 9504 9516 +12 Branches 2015 2015 =========================================== Hits 7444 7444 - Misses 1727 1739 +12 Partials 333 333 ```

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