Format and lint codes and fix packaging (#125)

* add: configure files (setup.py->setup.py+setup.cfg+pyproject.toml)

* add: __download_url__

* format with black and isort

* fix: flake8 section in setup.cfg

* add: E501 to flake ignore

* fix: metadata.name does not accept attr

* fix: merge __version__.py into __init__.py

* fix: flake8 errors in tests/

* fix: datetime.datetime -> datetime

* fix: banner

* fix: ignore W605 for banner

* fix: way to install deps in CI

* add: versem to setuptools

* fix: drop python<=3.6 (#126) from package and CI

tests/test_cdx_snapshot.py

@@ -1,4 +1,3 @@
-import pytest
 from datetime import datetime
 
 from waybackpy.cdx_snapshot import CDXSnapshot