Upgrade pylint and fix some warnings

[PiispaH]

No functional changes. Fixed some warnings from pylint.

Checklist before merging

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 54.85437% with 93 lines in your changes missing coverage. Please review.

Project coverage is 78.83%. Comparing base (e2c87bd) to head (b4a978a).

Files	Patch %	Lines
spinedb_api/parameter_value.py	38.33%	37 Missing :warning:
spinedb_api/spine_db_server.py	0.00%	23 Missing :warning:
spinedb_api/spine_io/exporters/gdx_writer.py	0.00%	8 Missing :warning:
spinedb_api/filters/value_transformer.py	33.33%	4 Missing :warning:
spinedb_api/export_mapping/settings.py	70.00%	2 Missing and 1 partial :warning:
spinedb_api/graph_layout_generator.py	0.00%	3 Missing :warning:
spinedb_api/helpers.py	25.00%	3 Missing :warning:
spinedb_api/db_mapping.py	33.33%	2 Missing :warning:
spinedb_api/import_mapping/import_mapping.py	90.00%	2 Missing :warning:
spinedb_api/spine_io/importers/json_reader.py	0.00%	2 Missing :warning:
... and 6 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #438 +/- ## ======================================= Coverage 78.83% 78.83% ======================================= Files 75 75 Lines 9888 9884 -4 Branches 2113 2112 -1 ======================================= - Hits 7795 7792 -3 + Misses 1745 1744 -1 Partials 348 348 ```

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