Improve packaging

[bonjourmauko]

Patch changes

• Improve pytest support for doctest
• Use pip check for dependency conflicts
• Add pyupgrade for automatic code formatting
• Add darglint for improved docstring linting
• Add .editorconfig for shared editor configuration
• Add pre-commit for automatic style checks & formatting
  • Git hooks are optional, as they may not suit everybody
  • To enable them, run pre-commit install
• Add long description to package distribution
• Update distribution keywords

Minor changes

• Relax upper-bound support for OpenFisca-Core
  • This package and OpenFisca-Core are mutually dependent, which breaks pip
  • The least astonishing is to assume this package to always be compatible with core
  • This is also easily enforceable thanks to regular dependabot checks that will run in CircleCI
  • Finally, it is the responsibility of the OpenFisca-Core integrity checks workflow to ensure this package to always be compatible
  • Every new version of core has to ensure all of the tests included in and distributed by this package pass
  • Also, every breaking change to OpenFisca-Core has to be phased out to help adapting this package to those changes

Major changes

• Drop support for Python < 3.7

[bonjourmauko]

Closing as stale.