Don't hardcode flask version for testinfra checks

[legoktm]

In 6e55b0548c2954ffbc6351a58c796dcaf4332b89 we bumped the the flask version but forgot to update the testinfra checks, necessitating b99469b8581fcdd7bc630ccd0487d4786d5512ed.

flask is the only version that's hardcoded like this, we should see if we can avoid hardcoding it and reading it on the fly from securedrop-app-code-requirements.txt.

[phershbe]

@legoktm @zenmonkeykstop I've been working on this but am a little bit confused. Do we want to read the Flask version variable from https://github.com/freedomofpress/securedrop/blob/develop/securedrop/requirements/python3/securedrop-app-code-requirements.txt and create a variable to be used any place we have a Flask version in the YAML files in https://github.com/freedomofpress/securedrop/tree/develop/molecule/testinfra/vars?