Upgrade transifex-client version.

This is only made in order to run the tests with a transifex-client version compatible with python3.8

The actual problem that we have in edx-platform is that the last version of transifex-client has as constraint python-slugify<2.0.0 which is not compatible with the current requierements of edx-platform on the other hand, the current version used in edx-platform is not compatible with python3.8