Installing elasticsearch-HQ I have cannot import name ‘Flask’ error

Hello,
Under kubuntu 18 I try to install elasticsearch-HQ,as written here http://docs.elastichq.org/installation.html#installation
But I got error :

pip install -r requirements.txt
…
Successfully built alembic
Installing collected packages: jmespath, itsdangerous, MarkupSafe, […]

See original post by invalid@example.com (mstdmstd)