Binarytree

- Fixed broken graphviz import

- Added support for `str` type node values. The values are compared using Python's default string comparators.
- Generator functions `binarytree.tree`, `binarytree.heap` and `binarytree.bst` now accept a new `letters: bool` parameter. If set to `True` (default: `False`), the generated tree nodes will have uppercase string values like `"A"`.

- Bumped up minimum dependency versions (e.g. setuptools, pytest, mypy)
- Deprecated support for Python 3.6
- Added [get_index](https://binarytree.readthedocs.io/en/main/specs.html#function-binarytree-get-index) function.

- Added new methods [binarytree.Node.equals](https://binarytree.readthedocs.io/en/main/specs.html#binarytree.Node.equals) and [binarytree.Node.clone](https://binarytree.readthedocs.io/en/main/specs.html#binarytree.Node.clone).
- Added functions to support another version of list representation (can be found in LeetCode): [binarytree.Node.values2](https://binarytree.readthedocs.io/en/main/specs.html#binarytree.Node.values2) and [binarytree.build2](https://binarytree.readthedocs.io/en/main/specs.html#function-binarytree-build2).

* Binarytree now works with Jupyter Notebooks natively (no dependence on Jupyter Notebook).

* Added Python type hint annotations.