Exit event handler if KeyEvent is of release kind

[amfaber]

Hello, I just ran through the MNIST example in the book, and I am really impressed with the project so far! Great balance between explicitness and easy of use.

A fantastic touch to have metrics shown in a tui by default as well, though I noticed that on my Windows computer, each arrow key press changed the metric being displayed twice - once on pressing and once on release. Due to the example having only two metrics, this meant that I needed to hold down the key and hope that I landed on the correct metric when I let go.

I just had a quick go at fixing this issue by early-returning from the event handler if the KeyEvent is of the Release variant.

All checks passed with run-checks.ps1 all

[syl20bnr]

@amfaber can you rebase your PR on top of main ? It should fixes the CI checks.

[antimora]

@syl20bnr rebased it myself. Lets push this PR through before the upcoming release.

[syl20bnr]

Thank you @antimora ! I merged it (although the tests are not passing) as the scope of the change is trivial and we know that CI is broken for another reason.