Clarify supported Qt bindings

[cdelston0]

At present, there is no documentation for which Qt bindings NodeGraphQt requires/supports. As a new developer, it would be really useful to know which bindings are being targetted by the package. Whilst supporting as many versions of Qt as possible would be a noble goal, it's an impossibly large testing surface to support.

I've done some experimentation here to try and determine which Qt versions are supported by installing the various options in a virtual environment and then running the basic_example.py to see if it starts at all.

I've found that that following options work:

PyQt5
PySide2
PySide6

PyQt6 gives the following traceback:

(nodes) chris@laptop:~/Projects/nodes$ PYTHONPATH=./NodeGraphQt/ python3 NodeGraphQt/examples/basic_example.py
Traceback (most recent call last):
File "/home/chris/Projects/nodes/NodeGraphQt/examples/basic_example.py", line 9, in
from examples.nodes import basic_nodes, custom_ports_node, group_node, widget_nodes
File "/home/chris/Projects/nodes/NodeGraphQt/examples/nodes/basic_nodes.py", line 3, in
from NodeGraphQt import BaseNode, BaseNodeCircle, BaseNodeSVG
File "/home/chris/Projects/nodes/NodeGraphQt/NodeGraphQt/init.py", line 50, in
from .base.graph import NodeGraph, SubGraph
File "/home/chris/Projects/nodes/NodeGraphQt/NodeGraphQt/base/graph.py", line 11, in
from NodeGraphQt.base.commands import (NodeAddedCmd, NodeMovedCmd,
File "/home/chris/Projects/nodes/NodeGraphQt/NodeGraphQt/base/commands.py", line 4, in
from NodeGraphQt.constants import PortTypeEnum
File "/home/chris/Projects/nodes/NodeGraphQt/NodeGraphQt/constants.py", line 38, in
ITEM_CACHE_MODE = QtWidgets.QGraphicsItem.DeviceCoordinateCache
AttributeError: type object 'QGraphicsItem' has no attribute 'DeviceCoordinateCache'

My humble suggestion would be that anyone picking this up for a new development is probably going to be starting with Qt6, so I think it makes sense to have a requirement on either PySide6 or PyQt6 (which will require fixing the above traceback...).

Thoughts?