Tksheet

Improved:
- Term searching improved when typing in a normal state dropdown box

Removed:
- `bind_event()` function, `bind()` to be used instead

Changed:
- tksheet no longer supports Python 3.6, only versions 3.7+

Removed:
Due to errors caused when using Python versions < 3.8 the following functions have been removed:
- __bool__
- __len__

Fixed:
- Sheet init error since version `6.3.0` when running Python 3.7

Fixed:
- Two `EditCellEvent`s being emitted, removed the one with `None` as `text` attribute
- Selection box and currently selected box in different places when tab key pressed with single cell selected
- Return key not working in dropdown box when mouse pointer is outside of dropdown
- Visual issue: Dropdown arrow staying up when clicking on same cell with readonly dropdown state
- Visual issue: Dropdown row/column all arrows in up position when one box is open

Removed:
- Many parameters from internal functions dealing with text editor to simplify code

Changed:
- Cell selection doesn't move on Return key when cell edit using text editor was invalid
- Events for `extra_bindings()` end cut, delete and paste are no longer emitted if no changes were made

Improved:
- Term searching improved when typing in a normal state dropdown box

Fixed:
- Cell editor right click not working
- Cell editor select all not working

Added:
- Some methods to `Sheet` objects, see [documentation](https://github.com/ragardner/tksheet/wiki/Version-6#sheet-methods) for more information.
- __bool__
- __len__
- __iter__
- __reversed__
- __contains__

Fixed:
- Incorrect row being targeted with hidden rows and text editor newline binding (potential error)

Fixed:
- [203](https://github.com/ragardner/tksheet/issues/203)