PR: Use raise from None when raising QtBindingsNotFoundError in __init__

[CAM-Gerlach]

As described in #390 , this fixes the raise QtBindingsNotFoundError in __init__,py to correctly use raise ... from None, to ensure it behaves appropriately and has a clear, understandable and non-confusing message.

Here's how the error output looked previously:

>>> import qtpy.QtCore
Traceback (most recent call last):
  File "C:\Users\C. A. M. Gerlach\Documents\dev\SpyderDev\qtpy\qtpy\__init__.py", line 252, in <module>
    from PySide6 import __version__ as PYSIDE_VERSION  # analysis:ignore
ModuleNotFoundError: No module named 'PySide6'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\C. A. M. Gerlach\Documents\dev\SpyderDev\qtpy\qtpy\__init__.py", line 259, in <module>
    raise QtBindingsNotFoundError()
qtpy.QtBindingsNotFoundError: No Qt bindings could be found

and here's how it looks now:

>>> import qtpy.QtCore
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\C. A. M. Gerlach\Documents\dev\SpyderDev\qtpy\qtpy\__init__.py", line 259, in <module>
    raise QtBindingsNotFoundError from None
qtpy.QtBindingsNotFoundError: No Qt bindings could be found

Fixes #390

[dalthviz]

Thanks @CAM-Gerlach !