Add version attribute and fix lints

Version bump

[skip ci]

Version bump

Version bump (#24)

Reduced demo GIF size below 10 MB (#21)

Closes #20.

Removed previous demo GIF

It is not shown on PyPI. Probably because of the comment right above it.

System packages in virtual environment (#19)

Allows using the pre-compiled wxPython package installed for the system Python interpreter rather than compiling it from source in the virtual environment.

Protect accessible attributes from modification (#16)

* Made `ScrollableFrameTk.frame` read-only
* Made `ScrollablePanelWx.panel` read-only
* Made `ScrollableAreaQt5.area` read-only
* Made `ScrollableAreaQt6.area` read-only
* Version bump

Added `ScrollableFrameTk` demo GIF (#13)

Human certification in readme (#11)

* Added certification note in readme
* Bumped version