Issue 14814: Make the ipaddress code easier to follow by using newer language features (patch by Serhiy Storchaka)

Issue 14814: Provide more informative error messages in ipaddress, and ensure that errors are caught as expected

Issue 14814: %s implies coercion with str() - remove a lot of redundant str() calls from the ipaddress implementation

Issue 14814: Eliminate bytes warnings from ipaddress by correctly throwing an exception early when given bytes data of the wrong length. Also removes 2.x backwards compatibility code from associated tests.

Issue 14814: Better handling of cases where octet/hextet parsing fails, including ensuring that tracebacks are still clean even when calling class constructors directly

Issue #15020: The program name used to search for Python's path is now python3 under Unix, not python.

Issue #15020: The program name used to search for Python's path is now "python3" under Unix, not "python".

Fix issue # 15033 - Return the proper exitcode for failure when modules are invoked using -m switch. Patch contributed by Jeff Knupp

Fix issue # 15033 - Return the proper exitcode for failure when modules are invoked using -m switch. Patch contributed by Jeff Knupp

working without _frozen_importlib by moving to an import over a direct
access in sys.modules.

Closes #14902: Fixed timezone conversion of a date/time in the past. Thanks to Yuriy Syrovetskiy for the report and Juancarlo Añez for the patch on which this fix is based.

Also eliminates some C code in Python/import.c as well.

Patch by Eric Snow with verification by comparing against another
patch from Jeff Knupp.

importlib.__init__, then catch the KeyError raised, not ImportError.

file size header field.

Thanks to Marc Abramowitz and Ronan Lamy for helping out with various
parts of the patch.