Python 3.8 support; Docker support

## [0.6.1] - 16/11/2020

### Added
- optional Dockerfile
- optional .env.docker for reference.
- Support of Python 3.8 is validated, see https://github.com/alex-ber/AlexBerUtils/issues/5

- Email formatting changed in Python 3.8, see https://github.com/alex-ber/AlexBerUtils/issues/7

Note that it is possible that `7bit` will be replaced with `8bit` as `Content-Transfer-Encoding`,
that I'm considering as ok.

- `check_all.py` to run all unit test.

- `check_d.py` for sanity test.

- .dockerignore

- requirements*.txt - dependencies version changed, see https://github.com/alex-ber/AlexBerUtils/issues/6
- Because of pytest upgrade `conftest.py` was changed:
`pytest_configure()` was added to support dynamically used marks.
- In `ymlparsers_test.py` deprecation warning removed (it will be error in Python 3.9)
`collections.Mapping` was changed to `collections.abc.Mapping`.

### Changed
- README.MD added section about Docker usage.
- setup.py to indicate support of Python 3.8