Remove dependency on pywin32 package on PyPy

[cgohlke]

Fix #226 and #229

This PR makes jupyter_core compatible with PyPy on Win32. The main issue is that the pywin32 package does not yet support PyPy (mhammond/pywin32#1289).

Changes:

1. add an alternative Ctypes implementation of the win32_restrict_file_to_user function, which is used as a fallback if the pywin32 package is not installed
2. change install_requires to exclude pywin32 on PyPy
3. allow AttributeError: 'stat_result' object has no attribute 'st_file_attributes' to pass silently

Tested with pypy3.7-v7.3.5.

[stonebig]

is there still a chance to get a jupyter_core-4.7.2 patch release with this in June ?

[stonebig]

it would be a small step for jupyter-core, but a great jump for PyPy visibility, as with the patch, Jupyter eco-system works on PyPy till seaborn, bqplot, ipyleaflet, matplotlib widget, bokeh, holoviews...

[minrk]

Thank you!