Move the project to the src layout to ensure we are testing on the
generated package rather than the checked out source tree.

This uses PEP 562 to implement lazy loading of submodules in dateutil
(GH-771).

Add and xfail unhandled case #1029

Migrate away from unittest and adopt pytest

Part of a series of commit that is moving the suite.

migrate away from unittest and adopt pytest for test_internals.py

isoparse: Fail with inconsistent time separators

Call .decode() only for binary data