Add flask 2 (<3.0) and flask 3 (>=3.0) to test matrix and do not fast fail

[foarsitter]

To get more insight if our fixes are working for both flask version I created a test-matrix for flask 2 (<3.0.0) & flask 3 (>=3.0.0)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a65dbe7) 96.15% compared to head (8bcd332) 96.15%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #577 +/- ## ======================================= Coverage 96.15% 96.15% ======================================= Files 20 20 Lines 2756 2756 ======================================= Hits 2650 2650 Misses 106 106 ```

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[peter-doggart]

Great idea! Hopefully the team over at pytest-flask will have an updated version soon. We should probably leave this in going forwards for a bit to ensure we don't break backwards compatibility too anyways though!

[foarsitter]

Looks like the problems with pytest-flask are solved since the Actions are succeeding :+1:

[cthoyt]

@peter-doggart it looks like this is working, I hope it can be merged soon!