Test against packaged version

[bonjourmauko]

Doing so allows to prevent publishing corrupted versions,
so that what we test is what the user gets when installing openfisca-core.

• fix: yaml tests bug #1048
• build: test built version country-template#158
• build: test built version extension-template#54
• build: add pyproject.toml, poetry, and tox #1015
• build: adapt package organisation #1059
• build: test against packaged version #1037