picotui: Add __init__.py package file.
picotui: Add __init__.py package file.

The original idea was to use "namespace package" which doesn't require
__init__.py. But namespace packages used in distribution packages have
various artifacts:
https://packaging.python.org/guides/packaging-namespace-packages/#creating-a-namespace-package

Given that "picotui" is a real package (not just a namespace for disparate
modules), make it such by adding __init__.py.

pfalcon/picotui