Fix VKeyboard missing with custom keyboard class

[Gobbel2000]

In window/__init__.py the VKeyboard class is only imported when needed, which only happens in request_keyboard(). If a custom keyboard class is set using set_vkeyboard_class(), it is never imported. In that case the function _get_kivy_vkheight() that was introduced in #7726 fails, because VKeyboard is still None. This change instead compares to self._vkeyboard_cls which is ensured above to not be None. If no custom class is set, this should still be just VKeyboard.

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

[D4SK]

This fix works great for me as well. I hope this will be merged soon. Seems like a very safe change. Maybe @DexerBR can comment as he added the change that is affected.

@misl6 @pythonic64

[misl6]

LGTM. Thank you!