this could happen if the application runs through the expiration time.

this is somewhat dubious, but seems like the simplest way for users to get
the needed data if it's not available on their system.

Upon module load, the validity of the available leap data is checked.  if
it's invalid (unavailable or expired) the latest leap data is download from
the IETF and stored in a local user cache.  This should only happen if the
data is unavailable (so first usage for mac or windows user), or expired
(which happens only once every couple of years).

This will find the appropriate location for the platform

This is needed to support older versions of setuptools that don't yet
support pyproject.toml.

closes #15

- add pyproject.toml for pep-518 builders (e.g. pip)
- fix broken setup_requires declaration in setup.py

which it can then join to form a complete date spec.

Setuptools will automatically adjust the interpreter for python
scripts.

global exclude .gitignore