Make it possible to type check gui

[eivindjahren]

Makes the top level gui module type-check

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] Commit history is consistent and clean, in line with the contribution guidelines.
• [x] Make sure tests pass locally (after every commit!)

When applicable

• [ ] When there are user facing changes: Updated documentation
• [ ] New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• [ ] Large PR: Prepare changes in small commits for more convenient review
• [ ] Bug fix: Add regression test for the bug
• [ ] Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.10638% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.03%. Comparing base (56dc74a) to head (3970b47). Report is 8 commits behind head on main.

Files	Patch %	Lines
src/ert/gui/about_dialog.py	0.00%	5 Missing :warning:
src/ert/gui/main.py	85.71%	1 Missing :warning:
src/ert/gui/main_window.py	96.96%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8086 +/- ## ========================================== - Coverage 86.04% 86.03% -0.01% ========================================== Files 382 382 Lines 23679 23692 +13 Branches 628 631 +3 ========================================== + Hits 20375 20384 +9 + Misses 3231 3228 -3 - Partials 73 80 +7 ```

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