Argopt

- drop `py<=3.6` support (18)
- code linting
- misc framework updates (18)
+ fix & update CI tests
+ fix release process
+ update & fix `pre-commit`

- misc framework updates (15)
+ fix CI tests
+ bump `pre-commit`
+ upgrade to `pytest` <= `nose`
- fix type handling
- minor sponsor link update

- support `partial` in `[default: ]` (14)
- fix potential exceptions & typecasting issues (14)
- update documentation
+ fix README example
- major tests, framework & formatting upgrades (14)

- framework upgrade
+ major build/dist/versioning system update
+ migrate Travis => GHA
- add pre-commit linting
- update documentation & badges

- fix `file` type in py3 (11 <- 9)
- minor framework updates & code tidy

- fix escaped chars in regex
- add `.[dev]` <- `requirements-dev.txt`
- update test framework
- complete CI framework overhaul
+ fix py26 tests
+ auto-release to PyPI