Q&D fix related to https://github.com/python-medikit/medikit/issues/108

[sei-nicolas]

I'm far too ignorant of medikit source code to know if this commit is relevant (so feel free to close/ignore!).

In the meantime, it seems that the root cause does not lie in medikit source code but rather in a mismatch between pip and pip-tools version, hence the modification on that point.

It's also possible that:

=> pip-tools ~=4.0 is fine with pip >=19.2 (not tested), but would require other adaptations to support pip-tools ~=4.0
=> pip will be corrected to fix discrepancies with pip-tools ~=3.0 in future release

[coveralls]

Pull Request Test Coverage Report for Build 500

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 47.182%

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Totals
Change from base Build 499:	-0.02%
Covered Lines:	969
Relevant Lines:	1848

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💛 - Coveralls