Release 5.2 blocked
The last version 19.1 of pip contains a major regression [1] which makes all CI tests fails [2].
This prevents to prepare release (or any push) with a correct feedback about successful tests.
So the release and any non-critical development are blocked until a working version of pip is released.
We are stuck with the last published version of pip because it is the behavior of tox to always install the latest version.
The CI can not be tuned to ignore a specific release of a package because it directly talk to pypi.org.
[1] https://github.com/pypa/pip/issues/6437
[2] https://drone.tryton.org/tryton/modules.account_invoice/176