Closes issue #20858: Enhancements/fixes to pure-python datetime module
This patch brings the pure-python datetime more in-line with the C
module. Patch contributed by Brian Kearns, a PyPy developer. PyPy
project has been running these modifications in PyPy2 stdlib.
This commit includes:
- General PEP8/cleanups;
- Better testing of argument types passed to constructors;
- Removal of duplicate operations;
- Optimization of timedelta creation;
- Caching the result of __hash__ like the C accelerator;
- Enhancements/bug fixes in tests.