Fix latest France web api to respect setup dependencies

[sandcha]

Delete upgrade eager install for latest api

Before:
This requirements.txt with this --upgrade option at installation upgraded Core version even when it was incompatible with France latest merged version.

[bonjourmauko]

Hello @sandcha, thanks !

• Could you explicit also the consequences ? (after)
• Do we have any trace of why were we using eager in the first place?

[Morendil]

@maukoquiroga I suspect the root cause is that pip doesn't have a clear story about how it resolves dependencies, see e.g. this issue elaborating on this classic. See also this comment shedding light on what would otherwise be a puzzling failure on openfisca-senegal.

My best guess is that we tried --upgrade but found it was not eager enough, now we've tried --upgrade eager and that's over-eager. No Goldilocks solution seems to exist in pip. The alternatives (we've previously mentioned Pipenv, zazo) seem to lack maturity.

So, it's possible that the consequences include "the next time we deploy, one of the dependencies stays behind instead of advancing as we wish". This is one reason we might want to consider deploying to a PaaS supporting an "immutable infrastructure" approach, installing from a clean slate each time.

[fpagnoux]

Arg, this PR removed the --upgrade, which means that we never upgrade anything... The /latest version of the API is now 8 majors behind the latest version of France...