Various fixes in wx.lib.calendar

[ricpol]

Fixes #1230

• wx.lib.calendar does not need wx.lib.CDate anymore. We just use datetime.date and wx.DateTime for all our needs;
• fixed a bug with key navigation across month boundaries;
• key navigation works fine now, setting the date and firing the EVT_CALENDAR;
• setter APIs works fine now, setting the date in a "smart" way (eg, automatically shifting the day when moving to a month with fewer days);
• a lot of fixes to wx.lib.calendar.CalendDlg, to make it work in the updated context;
• wx.lib.CDate should not be needed anymore and can be removed: however it is fixed now (the old py2 division operator caused rounding errors in py3);
• added/updated tests for both modules.

I tried to be as light-handed as possible, preserving the existing APIs. However there are still many annoyances and inconsistencies... In the long run a more in-depth rewrite would be required I think.

[RobinD42]

Please add a bullet item in CHANGES.rst in the 4.1.0 section about this change. Just a sentence to two can suffice.