Fix usage of database URLs in Data connection, Importer

[soininen]

This PR fixes a number of issues that prevented database URLs (other than SQLite) from being used as references in Data Connection and Source preview in Importer specification editor.

Fixes spine-tools/Spine-Toolbox#2329

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Attention: 51 lines in your changes are missing coverage. Please review.

Comparison is base (5a1edd5) 59.83% compared to head (7e6ddbb) 60.14%.

Files	Patch %	Lines
...ine_items/importer/widgets/import_editor_window.py	32.14%	16 Missing and 3 partials :warning:
spine_items/data_connection/data_connection.py	87.71%	6 Missing and 8 partials :warning:
spine_items/data_connection/utils.py	65.21%	7 Missing and 1 partial :warning:
spine_items/importer/importer.py	0.00%	6 Missing :warning:
spine_items/importer/importer_factory.py	0.00%	2 Missing :warning:
spine_items/database_validation.py	80.00%	1 Missing :warning:
spine_items/models.py	0.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #169 +/- ## =========================================== + Coverage 59.83% 60.14% +0.31% =========================================== Files 193 194 +1 Lines 17583 17651 +68 Branches 2858 2872 +14 =========================================== + Hits 10521 10617 +96 + Misses 6502 6469 -33 - Partials 560 565 +5 ```

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