skytools v3.9

Feature removal:

• Drop support for Python 3.6 and earlier.

Fixes:

• dbstruct: fix PUBLIC grant handling.

Cleanups:

• Apply mypy 'strict' typing to most modules and tests.
• Use pyproject.toml for project setup.

Warning: next release will drop some ancient and rarely used code:

• skytools.plpy_applyrow
• skytools.dbservice
• skytools.skylog.LogDBHandler