Force semantic versioning and release automatically

[fpagnoux]

Same than https://github.com/openfisca/openfisca-france/pull/468 for web-api.

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Tests :

• [x] Enforce version updating
• [x] Enforce CHANGELOG updating
• [x] Release tag
• [x] Release on pypi

[fpagnoux]

All tests ok, waiting for #56 to be merged, then just needs rebasing and squashing